Arrogant people. This is one of the top ways to get on my bad side, is cop a very arrogant attitude at me whenever you're informing me of something. This goes for people who say "You don't know anything about X because you haven't done Y." ...Seriously there are more ways than one to obtain experience or wisdom about things, and just because I didn't do it your way, it doesn't make me wrong or stupid. There's a reason why I opened my mouth. XD
A related peeve of mine is people who refuse to listen to reason. If you ask me something, expect to hear me being brutally honest. (Sometimes you don't have to ask, but if I am asked, I never sugarcoat it the way I might if I weren't asked directly.) If I'm trying to talk through a conflict, nothing pisses me off more than refusing to listen. Once I'm pissed off at you for that, I turn quite nasty sometimes, as it's one of the few things that can make me smash a stack. I'm a caring person, so when people refuse to hear me without properly explaining their side, it gets me pretty steamed because I'm a stubborn person. xD
Excessively immature reactions
This is another quick way to get my tongue forked and me spitting poison. XD
Overreacting almost always magnifies problems, and this one ties right into the previous two peeves of mine. I won't tolerate excessive immaturity when you've shown a respectable level of maturity, unless we're just joking around or having fun.
Joke-sults (Joking insults)
This one ties into the previous one. Now with that being said, I can tolerate a few barbs thrown at me in good humor, as long as they're not worded extremely offensively, but I expect all of my friends to have a certain level of tact. Usually if I'm not in the mood for these barbs, I leave little signs of that in my manner of speaking, behavior and such. I do this intentionally...and for you denser types, I even tell you when I'm not in a good mood, so it's not like you aren't suitably warned.
This can quickly land you on my bad side if they're pointed at a friend of mine, if I don't know that you share a close enough friendship for it to be tolerated, so make sure you introduce your friends to me properly and let me know about how close you are in friendship so I don't misinterpret things. (I'm fiercely loyal to my friends once the bond has deepened enough, so keep that in mind.)
Bullying
I can't stand to see bullies, and this does tie into the previous peeve, as well as the previous one ties greatly into this one. I wasn't popular as a child, and I simply won't tolerate that behavior from anyone. If I can't tell you off or do something to punish you for it, I will strongly dislike you for a long time, as well as persist in ways to make you absolutely miserable. Naturally, this also ties right into my peeve about extreme immaturity. It's quite immature to bully others for any reason at all. I respect people who try to resolve their differences peacefully.
Stuck up people
People like this are the worst. Not only are they immature, but they're arrogant and they're bullies to boot! You may never get me to like you once I've decided you're stuck up. XD
(Quite simply, your chances are slim unless you do a complete 180 in plain sight)