You know you want to do it, so get it out of your system.
1. Lag as seeing the game slow down doesn't happen much (not even in 0 bar connections) but INPUT LAG is a bitch. When my opponent does a move that leaves him at a huge disadvantage if blocked and I block, try to punish and get counter hit instead...It makes me mad...
2. The ranking system. I ranked up to Barbarian and then down to Hunter again. I never reject rematches whether I win or lose but many people seem to be cherry picking their way to the top, as they reject rematches if I win. I have taken my win rate from 35% to 50% and I have gotten demoted LOL. I will continue improving but ranking up is another story.
3. People who get salty when they lose. I had never in my life gotten so many angry messages. One guy even sent me a lot of voice messages trying to explain why I was cheap and he had skills.
The piano has been drinking...not me.