Personally, nothings has given me more trouble than Steam. I hate it. Why can't there be a better game managment platform?
Other infuriating things that rank high in my list include:
Hypocrites
People that intentionally piss you off even though you've done nothing to them
People that act like they're entitled to things when they're not
Gangnam Style/Harlem Shake fanatics
Swagcigarettes/Yolo people that don't say it jokingly and really go through with the craziest things
The majority of tween girls
People that say 'omg', 'omfg', 'lol', 'ttyl', or other junk in that vein in real life
British superioritists
Female "equalitists"/superioritists
People that shove their opinion down your throat (also includes gay people that shove their rooster down your throat)