My biggest professional mistake occured two weekends ago as a second shooter. The main and I were outside shooting the B&G, and his card was full. I pulled the card out of my 2nd body, which I had used for some of the formals at the church and some of the ceremony. Unfortunately he didn't see me pull it out of my camera, and I wasn't really thinking about it when I handed him the card. Naturally the first thing he did was format the card and go on shooting, and I didn't realize it until after he shot with it for half an hour so recovery was likely not possible.
Thankfully I had shot the procession, kiss, most of the formals, and the rest of the critical stuff with my primary body. We redid the formals outside without even telling them, so we pretty much managed to cover our butts.
Current: 5DM3, 6D, 8mm fish, 24-105/4IS, 35/2IS, 70-200/2.8IS, 85/1.8, 100-400/IS v1, lensbaby composer with edge 80, 580's and AB800's
Formerly: 80D, 7D, 300D, 5D, 5DM2, 20D, 50D, 1DM2, 17-55IS, 24-70/2.8, 28-135IS, 40/2.8, 50/1.8, 50/1.4, 70-200/4IS, 70-300IS, 70-200/2.8, 100 macro, 400/5.6, tammy 17-50 and 28-75, sigma 50 macro & 100-300