We did have one day of real winter here this year so I get to chime in with my one trip experience. My big surprise hazard: chain frozen to the cogs and rear derailleur! I got overconfident and up-shifted, not realizing that the chain moving over the gears was the only thing keeping the ice off. Chain sliding over, then frozen solid when I stopped. You can't shift, you shouldn't stop, who knew?

56) Iced up drive train

Darn, Alan S beat me to it post number 22, so this should be 35) freezing drive train