What characteristics in people do you find the most utterly repulsive and irritating?

For me it's when people are flaky. My friend is such a flake, that sometimes I just want to stab her. Once I asked her to pick up my weed from her roommate (this guy I buy my weed from) and to give it to me when we meet up. So we never ended up meeting because she was flaking. I told her to hold on to my shit for me until I saw her later. So the next day I call her to hang out, she says okay. Then I ask her about my weed and she says "oh I kinda smoked it all". I just wanted to blow up. (I was only significantly angry because she is so selfish and careless) I told her that was messed up and she said she'll get me some to make up for it and I said okay. She never gave me some and it's been over two weeks. She didn't pay me back for some money she borrowed. She always calls me for rides. She always says shes going to call me when she gets home to hang out, but never does and I find out she went out to the bar with other friends. She is so damn flaky and I really just want to punch her. I haven't told her anything.

tl;dr: my friend is so fucking flaky and I want to rip her head off.