I think we've all forgotten to add priming sugar at least once. My biggest flaw is that I forget 75% of the time. So far I've gotten lucky and caught it after 1-6 bottles each time. No blood, no foul.
Honestly though, my biggest mistake (and I think many will agree with me here) is not doing a dry-run first when I get new equipment. It always takes a few batches to dial in your process, and as has been said before, that resets with each change to your equipment. My first attempt at a full boil with a turkey fryer met with near disaster due to constant 10-15 mph winds in my area. HURR DURR! Since then, I moved to the garage and never looked back.
Another all too common mistake is complacency. Well heck, my last few batches didn't boil over when I used a fan to dissipate surface tension, so this one won't either. Ask my wife how that one turned out, or walk bare-foot in my kitchen to learn first hand.