One of the biggest mistakes that could've turned out bad for me was being in too much of a hurry to make it to work on time on a rainy morning. I took a turn too fast and tight and ended up going for a ride on the ol' asphalt slip 'n slide across half a lane. Luckily, the cars at the intersection whose lane I slid into were stopped at the light. I only took the risky turn 'cause I knew they weren't coming, though so I guess it wasn't super stupid. My tires were also pretty worn at the time.

That's just riding in general though. For commuting, definitely it would be not having a spare inner tube or patch kit. That usually leads to either calling a friend for help or a walk of a couple of miles.