I didn't coordinate rainwater leaders correctly on my first project.  It was a school.  Eventually a hole was made in a steel I-beam to allow the RWL to pass through  

I have never made that mistake again.

Uneven loading of exits.  Ended up having to fix a shitload of hardware and doors to make up for the additional load placed on other doors when it was discovered.  I didn't do it.  Just had to deal with the aftermath. 

If posting, make sure any relevant statutes of limitations have expired  ;o]

Not mine but on a 8-story office building, the GC recommended a sticky plate with nail to the PA as way to hold the mineral wool to the exterior concrete walls.

System gets approved, the wool goes up (8 stories), the cladding is on hold, then the rain starts.  Oops, there goes a shit-tonne of wet insulation.  

My personal mistake was not including a critical min clear dimensions on stairwell to accommodate form work tolerances. Turned out to be an expensive and complicated fix.  Never again will I set up critical dimensions without following through on shops and with extra fuck-up cushion.     

A licensed project architect at my previous firm provided the wrong issue status on a CD package for a government project. As a result the submission was rejected. It was just 2 letters and she got it wrong! WTF. 

I don't make mistakes (period).

This probably hasn't been the biggest mistake, but one that I vividly remember from my first job. I had detailed the stairs and railings on a project making sure we had the right clear width, landing size, etc. Turns out I screwed up drawing the stairs so the top of the stringer was extending past the railing at the landings. Perfect place for everyone to trip on it in an egress situation. It was caught in the shops and didn't take much to correct it, but I thought for sure I was going to get fired or have it come out of my paycheck. PA just laughed at me and said not to worry, mistakes happen.

Probably the worst was - very early in my career - I trusted a Revit output to calculate FAR, without verifying the area boundaries. Turns out voids in the slab (i.e. every stair run) was deducted from the area. 

Nobody noticed until mid DD. I was sure I was going to lose my job over it, but we scrambled to re-design the building in 4 days and then a month later the entire company went out of business anyway. 

I'm not sure I can say more than something collapsed. I was just 3 years out of school, made a mistake and while there were many opportunities to be caught in engineering coordination and shops, it wasn't. Was not occupied at the time of collapse so one was hurt. :)

so far, my big mistake was to receive a verbal approval for a shower tile, and not ask for it in writing; 3 weeks later, owner comes in, doesn't like the tile and blamed me for it. redo the whole shower. she paid for it anyway.

Not necessarily at work but one of my greatest regrets was turning down a government entry level position to work at a private firm. I did not know it at the time but the principal of the private firm was a major dick who regularly threw tantrums and worked his employees to the ground w/o extra pay. Eventually, I just did not give a rip about anything at that job and got fired. I got some good unemployment money though.

I moved across the country to take a job with a great designer, and as it turns out, a pretty big turd of a person and an even worse businessperson. No regrets, but that was a mistake. 

During preconstruction I wrote an RFI that a multistory riser housing almost all the ducts (supply, return, kitchen exhaust/grease) was far too small to fit the scope. The engineer refused to respond to it until the mechanical contractor was brought on board months later. Turns out, not only was it not big enough, but not to code. The engineer wasted an additional month or so insisting we were wrong and refused to accept any alternative solutions the GC and trade contractor proposed that met code and their design intent. After 4 months, they finally accepted the first of 4 alternatives we had proposed the first few weeks after the trade had been brought on board. Their refusal to admit they made an error, fix it with our help, and move on, delayed construction by 4 months.

my wife's office (not her project) for a multi-building residential development, north arrow rotated incorrectly for plans of one building type. foundations were done before someone noticed things weren't where they ought to be for that block. the firm paid out of pocket to fix that! bad year for bonuses :(

Stairs seem to be a headache way too often. Nobody ever seems interested in making floor-to-floor heights align with a uniform riser. Clearances are forgotten so they end up too small with no room to grow. And then, even when they’re good to go, you get the old principle lecturing you on outdated code because you don’t have the 12” horizontal railing extension at the bottom.

Ramps also seems to be frequently undersized. Actually, way too much dealing with accessibility seems to start off undersized.

Oh, I almost forgot this one.

The GC set the crane base's location by scaling our site plan drawing... and we discovered that only when 3 or 4 stories above grade.  The building was only 50mm off on one side and 150mm or so on another but everyone was very quiet while I tried to figure shit out.  

I had three terrazzo floor patterns in permit/ bid set and client took one out and swapped for another when bids came in low. Was supposed to be the same pattern but different metal for divider strips. I just replaced the tags but didn't fix the metal color. 

They got like 30' down this 20' wide space before I caught it. All the terrazzo poured and cured, nothing to do but jackhammer it out. In my defense I did have a note there and in the specs that the architect should be given 24hr notice to inspect the divider strips layout in person before the terrazzo was poured so my firm didn't have to pay for it since we would have caught it if the GC followed the notes. But still my mistake.

At another firm, a mistake on a zoning calc led to a multi-million dollar lawsuit from a developer we were working for. They were seeking to recover all the lost revenue for the units we told them they could get but which weren't actually possible per code. Glad I wasn't on that project....

Not my mistake, but had a project in the office where the designer over detailed the stairs to make sure they got certain design elements right. The spec was written to delegate the engineering of the stairs to the GC, but since it was over detailed and everything was dimensioned to the nth degree, the stair fabricators just copied our drawings for their shops and built it like the drawings showed. 

It wasn't until it was in place that someone realized that no one had actually engineered the stairs to carry the loads. Structural engineer took the shops, did the calcs, and with a few minor field modifications they were able to carry the loads. Spearin doctrine would have screwed us if it would have been an expensive fix. 

Also a good lesson for understanding basic rules of thumb and approximate sizes of structural members because the designer was actually really close with just their intuition. 

I have seen lots of screw ups over the years, but have only seen one person get canned directly because of it. 

It was a curtainwall that had multiple color finishes that were to be applied to different elements. Approved shop drawing got the patterns all wrong. Thing got built. Architect who approved the shop got canned for "poor attitude". They were complaining about having to work over the weekend trying to figure out the fix. 

Client did not care about this at all. They were fine with what was built. Head designer was the one who was really pissed. 

A few years ago, I approved the wrong paint system - it was ok for the substrate but not what was specified - the GC complained that it was more costly and it definitely ended up adding time on their project.  I don't really understand why they submitted this system if they knew it was more expensive, going to add time to their project, and it didn't fit the spec.  They submitted 3 different systems all at the same time so I was unclear about which one they actually intended to use.  But either way, I probably shouldn't have approved it!  

Same job, there was an issue with the paint stripper.  The job had concrete kneewalls with a failing elastomeric coating on one side.  For whatever reason, no one did testing before CDs were completed so we specified products we knew had worked on a previous job.  We did testing with a rep present during CA, but none of the products we listed worked as expected.  The product we went with required 7 rounds of application (!) before we got to anything close to bare concrete.  Thankfully, the client was somewhat ok with extending the contract to cover both of these issues.

this nice part about this job is that things are very seldom solely your fault.

I did a tenant fit-out in the ground floor of a new small mixed-use condo building. Typical podium construction with wood residential on top. The building happened to be designed by a different team in my office, in conjunction with an out of state architect who designed and documented the upper floors, which were modular construction. Well, at some point the developer decided that the podium was too expensive and decided that the modular units would sit directly on top of the podium steel. They added 2 layers of gyp for the fire rating and called it a day.

In the middle of construction for the fit-out, it was discovered that the assembly that sat on the steel did not meet minimum required STC. At that point, the GC had installed all of our lighting, duct work, and had painted the ceiling. We had to take everything down, install a suspended Chicago grid w/ (2) layers of gyp to make the rating. It cost a LOT of extra month and added a month to the schedule.

The really frustrating part is that even though I had no role in the design of the building, I felt that our firm was partially responsible for this issue. Even though the assembly was designed by the out of state architect, we were the Architect of Record. None of the senior management took any responsibility for this. They blamed it on the developer (who is an ass) and the out of state architect.

Not the worst mistake but the best story. I learned the hard way that there is a Benjamin Moore primer where the product number is the same as the color number for Benjamin Moore 'sunflower yellow' paint. For this particular project, the GC didn't submit the paint for approval prior to applying. So they painted the whole space yellow and then tried to blame us for it.

...this happened twice at two different firms. 

In my defense, the second time it happened it was a teammate's project.

My biggest mistakes have been extending trust and good will to people who don't deserve either. 

On one of my first projects I failed to specify what should have been common practice, and would have been with any experienced builder. In a house renovation, exterior sliding glass doors were installed with headers bearing on posts bearing on a single box joist. Of course the joist should have been blocked solid beneath the post for bearing. 

Some years later some repairs were required due to settling (crushing of the joist) ... which didn't bother me quite as much as it should have as the owners shorted me on the final payment. But I still feel bad enough about it to share the story.

Two lessons here: call out bearing to foundation as necessary AND inspect to see that it is done.

I had a similar situation to Miles', for a 14' wide double sliding aluminum-clad door. I told the contractor multiple times to install squash blocks at the rim joist but he never did, and they weren't on the plans. Plus space was tight and I didn't plan for enough gap at the top of the door; even though I designed the beam for L/480 deflection, that was more than the gap allowed . The result is a $15K doorway with doors that bind and no good way to fix it.

Another time I designed a kitchen renovation with a glass-door Subzero in the middle of a big span. The existing joists had been fine for 40 years but the added weight made the floor sag, resulting in cabinets pulling away from the ceiling. It was an easy enough fix but embarrassing at the time.  

One more: a couple of years ago I designed a house with a truss roof spanning over 40'. I told the contractor to plan for truss uplift and even sent him articles on how to deal with it, but did not specify anything about it on the plans, and trusses are not common on higher-end homes in my area. The kitchen is in the center of the house and there is a 1/2" gap above them in the winter, that closes in the summer. 

At least I've learned my lessons on each of those oversights, and have not made the same mistake twice....