Reddit top voted answers...
The fatal combination of stupid AND rude.
When someone blames me for something I clearly did not do.
People who aren't interested in listening to your side of an argument.
Hitting my head on something.
When people start saying their problems are worse than yours and that yours don't matter as a result. Everyone has problems, they don't cancel out, asshole.
The moron that stops in a roundabout.
The moment you go to click on a link or story and an ad loads right as you touch it, sending you to a page full of virtual AIDS.