Groups who take up the entire sidewalk/hallway, yet walk at seemingly the slowest pace possible. Bonus points if they call you rude after you politely ask them to move out of the way.
Oh my god, yes, this! I like to try and get some exercise and de-stress at work by walking on my breaks. Lots of other people do the same thing, but there are limited places to walk, even outside. The courteous thing to do when someone comes up behind you walking faster than you is to move to one side and let them pass. But I've seen single people, often on their phones, weaving from side to side blocking me no matter which way I try to go around. And if they have earbuds in, they don't even hear me saying, "Excuse me."
Another pet peeve is not keeping the public restrooms clean. If you want to hover, I understand, but can you at least wipe up the splatter after you're done. Sometimes it doesn't even look like splatter. It looks like they went directly on the seat. And my workplace is really bad about women leaving blood behind. That is the ultimate in disgusting!
Also, I can't stand it when people ruin a movie for others. Talking, kicking seats, being on their phone, and just generally obnoxious behavior. I have literally seen people on opening night turn their backs to the screen and hold a conversation with the people behind them through the entire movie. I paid a lot of money for my tickets and I don't get to go out to the movies often. I'd like to enjoy it.
Finally, people who let their dogs roam around off-leash in a populated area. Their dogs come running up to us when I'm walking my dog and their owner just smiles and cheerfully yells, "It's ok. He/she is friendly." Well, my dog isn't. My dog, who is properly leashed and under control, is terrified of other dogs and may potentially bite if he feels cornered. So I have to scramble to put myself between them or pick my dog up so that he doesn't do something that could get him in trouble, even though in my opinion it's the other person's fault.