What's the most embarrassing thing you've ever done on the job?
I was wiring the light switch at the new spa at the Luxor in Las Vegas. When I flipped the switch, all the toilets in the restrooms flushed.
DOH! 
Come on, fess up. Let's hear 'em
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Sure, we've all had stupid bosses... until we become the boss. Then we're the smartest guy in the room.
Tell me I'm lying. 
I was sent out as the crew chief with a brand new principal investigator (PI) with a fresh master's to show him the ropes. One of his responsibilities was to do the background research to identify known (recorded) sites (like burial mounds) before we went in the field. I was out running another job while he did that. Long story short, I got preoccupied recording some stuff with the GPS, he was off doing who knows what, and my crew ran a transect of shovel tests right across a burial mound that I had not recognized, but was recorded.
In the end, I had to go explain/apologize to a inquiry by representatives of several tribes (Iowa) along with our project manager. The new PI quit (taking his office product with him, such as it was) rather than face the music. We ended up having to undergo retraining on the identification of mounds, but really it wasn't that bad, as they respected our forthrightness and acceptance of culpability. I don't think the PI had much of a career, though. Maybe he went back for a doctorate. 
Oh, we did not disturb any human remains that we could identify. So that was good...
At one job, I was having a lot of stress, so at lunchtime one day I went out for a lesson in progressive relaxation and meditation.
When I got back to work, I was so relaxed and uncoordinated that I ended up flushing my sunglasses down the toilet in the ladies room. Had to call our janitor and explain to him what happened. He somehow managed to get them out and kindly asked me if I wanted them back. Uhhhh....no.
At another job, it wasn't what I did, but what someone else did...the boss was out of the office for a bit one day, so there were only the bookkeeper and myself there at lunchtime. The UPS driver showed up and asked if he could use our bathroom (the only one we had) so we figured he's going to take a pee and leave.
NO!
He had a nasty case of diarrhea and stunk up the bathroom and the inner office and then the smell drifted out to the area where the bookkeeper and I were. The bookkeeper left after lunch, and I was there alone when the boss came back and, smelling the disgusting stench, assumed I had done it. Of course, I couldn't tell him that the UPS guy was responsible because he wouldn't have liked that we let someone else use the bathroom.
sigh...
.... I wore mismatched shoes (same style pumps, just different colors).
Also in the good old days before firewalls, was chatting with a gent on Yahoo Messenger using my (rather racy) screen name, got an IM from a co-worker and forgot to flip back to my real name when I replied to her.
I was a maid at a hotel. Our boss insisted we come in a 7 am, even though we were the weekend crew and people checked out later on the weekends. For the first few hours, we had hardly any rooms to clean, so we'd go around knocking on doors to see if anyone who hadn't marked 'check out' had left (it usually happened that half of the people would leave without officially 'checking out' on our papers). Our supervisor hated us waiting around for rooms to check out, so we had to be proactive and look busy. So stupid, I know, I told them how dumb this is and how it would be so much easier if we just started work at 9 am instead of 7 am. I was just a dumb teenage maid, what did I know?
So, my partner and I (We worked in pairs) listened at the door of this one room that was between 2 check outs. We didn't hear a tv. We didn't hear a shower. We didn't hear any banging around. So we knocked and loudly bellowed, "HOUSEKEEPING!". We waited a few seconds. Knocked and bellowed again. Nothing. One last time, "Houuuuuusekeeeeepiiiiiiiiiiiiiiinnnnnnnng!!!!!!"
Nothing. So, I turned the key and opened the door ever so slowly. I peeked around the corner. Nobody there. I looked the other way. Coast was clear. I motioned to my partner to come on in, looked like the room was empty. So we flung the door open and walked in. Just then a guy who was butt naked came around the corner. OMG! We got out of that room SO fast. Of course being teens we were SO embarrassed but laughed our asses off.
Let's just say after that, we were allowed to go to the coffee room to wait for more check outs if we had no rooms left that were checked out to clean. And we got to start at 7:30 am after that. 
