The most irritating thing about low elo.
Every game is a huge snowball. Once a lane starts losing, they refuse to change anything about how they play, or what they build, so lanes that get a bad start tend to feed and snowball the entire game beyond what is carryable. You can play your lane well, get ahead a bit, not feed, and try to help the losing lane, but you can't force the person to build a single defensive item when they get oneshot in half a combo.
It can happen on either team, but in an otherwise evenly matched game, people who just refuse to build a single defensive item, or overextend for cs after already dying 3 times to the enemy is just so ungodly frustrating, esp when they start flaming other people on the team for their own mistake.