The one that ensures the copier will jam when you're urgently need it.
Cars have this to, I'm sure of it. Do everything you can to prepare for winter, it's all in vain. When it snows and its cold, that's when the blasted car will blow a waterpump or such.
Grr.
Hey, then again, maybe if I build a heated garage, that will prevent the gremlins, because I wouldn't be cold and misurable fixing things outside in the dark.