Stupid people.
Cold weather.
Thing that don't work for any obvious reason.
People not making sense.
People who don't follow logic when explaining things.
Drivers who either stop unexpectedly, get too close to me, or dither after stopping.
Thugs.
Bad spelling/grammar.
The way my cats know EXACTLY what they want, but can't communicate it in any readable form.
Times when there's nothing on TV.
Dirty/wet/steamed-up glasses.
Not being able to distinguish between the two layers of T-shirts I'm wearing when trying to clean my glasses.
Rain.
Ice.
Gravel paths.
Huge sig images.
Badly-maintained engines.
Wet roads.
Muddy paths.
Lack of grip.
Skorpynekomimi [FGTL]
Nyaow!

What scares me? Mobile phones, bad driving, and brake lights.
(Spend some time drafting traffic on a bike and you'll understand the last one)