Mine is, not finishing Games : (
I've made plenty of mistakes, but if I were to pick one, it's not making proper backups. I'm better about that now I'm using github regularly, but for example I lost the FBX exporter that I kludged into the original HyperBowl game in order to export its data out to Unity. I spent at least a month on it and it was so messy I told myself I'd make a backup after I got it cleaned up, but then the PC crashed and I could never summon the motivation to reimplement it to fix some export glitches. Studios make this mistake, too. I've been at places where they didn't bother to keep anything around from previous projects even when it could have been reused.