2 (among many boners that I've pulled) that stand out in my memory:

I sent a couple of guys to "Make safe and demo" an office area in a 2 tenant warehouse building that was vacant. The new tenant would be getting some new offices.
The next morning the superintendent called me and asked what had happened. The guys hadn't done a thing.
I called our electrician, who told me that he'd completed the job and already dropped off the paperwork at the shop. Then I pulled the work order up on the computer.

-I had sent them to the wrong side of the building, and they'd demoed about 2500 sq ft of office that was to remain....oops

soon afterward

I ordered a 625' pull of 750kcmil aluminum, however I failed to write aluminum on the order. This happened about a month after I had told our inside salesman that he didn't need to ask if I wanted al or cu, "Just send copper unless I specify otherwise"
Fortunately, they took back the 4 reels of 750 copper, so it wasn't expensive.

But it sure was humbling.