I think the biggest one for me would have to be when I was making chicken cacciatore for a wedding I catered. I gave it to one of my staff to look after because I trusted him to get it done. He took on the chicken and had other, inexperienced helpers doing the chopping. It took them forever to get the onions and peppers ready and my staffer seared the meat well, but when he put it in the oven he didn't allow extra time for it to cook as the original recipe said 20 minutes (for 8 servings in a skillet on top of the stove). We had to do it in the oven which takes much longer. My bad was I never checked it and several servings were reported as being "pink". I had to pull the dish, nuke the chicken, put it back in the sauce but by this time no one wanted to eat it. I ended up refunding the amount of the chicken on a wedding I really wasn't making much on anyway.
What did I learn? Even if I trust my staff, always do my own quality control. It was a moot point, however, because that was the last event I did as I put my leg out in the middle of it and realized my body had had enough! Now I just consult and do cakes.