Well, Im a newbie so I havent had much time to do very many really dumb things yet, but I managed to pull a pretty dumb and embarrassing one during my 3rd OW certification dive.
We were at Cypress Springs in Florida, and there were 12 OW students (including yours truly). Our dive instructor split us up into 3 groups of four so she could manage us easier. This day I was in the last group. In order to speed up the process the divemasters had each subsequent group waiting on the surface so that when the prior group surfaced, we could immediately move into place and start our descent.
I was waiting patiently at the surface just before the second group surfaced when much to my horror I realized I forgot to put on my weight belt. Of course this meant that my buddy check..... BWRAF.....was totally skipped. It was too late for me to exit the water and go running after it, so I had to announce it to everyone and have one of the divemasters go retrieve it for me. To my credit though, I stayed in the water and put the wieght belt on as I was trained to do, and the divemaster complimented the fact that I handled it well.
It sure was embarrassing though.....and the experience reinforced my resolve to ALWAYS do my buddy check in the future!!!!