Updates that reboot your PC mid-game
Unexplained internet outages / slow speeds
It's Saturday night. After spending the past two days hanging out with your BFFs, you're ready to dedicate this entire evening to your favorite online game. A two-liter of soda sits atop your desk, a hot pizza on the table nearby, and some energy drinks reside in the fridge for when sleep attempts to deprive you of joy. An hour into your perfect night, your internet connection takes an dump. Maybe it cuts out altogether--or worse, it works, but so slowly that you can't play without rubber banding all over the freakin' place, and OH GOD just thinking about this is upsetting.
Getting kicked from from a game for "being a bad"
There are few things as frustrating as being booted from a dungeon run in an MMO because you were still learning the ropes, or reported for intentionally feeding the opposing team in a MOBA like League of Legends when, in reality, you didn't know what the hell you were doing. It takes a little bit to learn the ins and outs of even the most basic of competitive games, and getting penalized for trying to do so just plain sucks.
Getting kicked for being too good
Then there's the opposite end of the spectrum. If you've become an amazing player in a particular shooter after spending dozens of hours honing your skills, or just lucked into an unprecedented winning streak, people tend to assume you're too good. So good, in fact, that they believe you're cheating the system. Next thing you know, everyone's accusing you of being a "hax0r," and demanding the admin to perma-ban you from the server. Just when these complaints are flowing in full force, you nail a seemingly impossible headshot--it was luck, honest!--and the admin's corpse falls to the floor. PERMA-BANNED.
Online teammates that rage quit
It's incredibly frustrating when things just aren't going your way, especially in an online match. And when your team relies on each member to pull his or her weight (especially during games that don't allow substitutes to fill a vacant spot), it can be absolutely devastating when someone just up and quits because their rage meter has reached its limit. There's no "I hate you guys" in "team."
When the person in front of you at a store gets the last copy of a game you wanted
The newest game in your favorite franchise just released, and you've been anxiously waiting for school/work to end so you can go pick it up. It's a popular series, but retailers in this day and age typically have more than enough copies on hand. Or at least, that's what you think until you arrive at the story and patiently wait in line, only to find out that the guy in front of you just bought the last copy.
Gold farmers
Gold farmers are the bane of all MMOs. Their chat spam ("hello!, come 2 visit gold4cheap.com for 1000g/$7.99, coupon code #1414 BUY 1 GET 1 FREE $$$$$!!!!!") will quickly make you want to sever all communication with public channels, which kind of defeats the point of a massively multiplayer online game. At least you can derive some sort of joy from hammering away at the report option when things get real bad.
Achievements / Trophies that spoil stuff for you
Hunting for Achievements and Trophies is an addicting hobby for many gamers. For others, they can sour the whole experience. How many times have you downed a boss, not knowing if it was the final enemy or merely a prelude to something greater, only to be showered with multiple Achievements congratulating you for completing the game? This scenario is a huge bummer when you know, without a doubt, that there's nothing else in store aside from a brief ending cutscene.
Multiplayer Achievements
Ah, yes, multiplayer Achievements--a game feature that no one asked for. Man is this a huge pain, especially in games where the multiplayer component is tacked-on garbage. It's infuriating to be one or two Achievements away from 100-percent completion, only to find you're roadblocked by one that requires you to win 8,000 matches of team deathmatch. Which nobody--nobody--plays. How convenient.
People who write off entire genres as garbage
Let's say you have a friend name Billy, who really likes video games of all genres. Well, most genres. See, Billy doesn't particularly understand the finer points of resource gathering and base building, and has convinced himself that real-time strategy games--your favorite type--are dumber than Nintendo's investment in the Virtual Boy. "But Billy," you protest, "there's so much skill involved! You have to micromanage, learn when to invest resources in passive upgrades or new units, develop strategies for taking on an equally intelligent opponent. There are so many elements at play!" To which Billy, unwavering in his opinion, responds: "You're an idiot, as is anyone who plays those stupid games." To the Billy's out there: just because you hate a genre doesn't mean others need to adopt your opinion as fact.
