Sitting in the middle or 'fast' lane of a motorway when not overtaking.
Motorists stopping at traffic lights in the bit reserved for bikes.
Cyclists who go slowly but skip all the traffic lights, so I have to overtake them (on my bike) several times because they go past me at the lights.
Cyclists or swimmers who don't like being overtaken so deliberately speed up and re-overtake only to slow down again afterwards.
^ Basically, a lot of stuff to do with driving & cycling.
Also:
Linguistic prescriptivism, but also sloppy writing and grammar.
Smoking.
One particular person in my extended family who uploads (literally) several hundred photos a day to SM but doesn't get the concept of hashtags so creates new hashtags which don't link to anything except her own post. Posts have several other annoying features such as bad spelling, cutesy nicknames which really should remain private, far far TMI about things like children's bowel movements.
Myself for being such a bitch about family member above.
'I bet no one will share this ...' posts implying that I don't care about people suffering from cancer, depression, or whatever unless I share the post.
I mean, there's a lot of more important stuff too, to do with politics, or everyday sexism, or the perception of religion, but I kind of think those things are my business so they don't count here.