When your teammate keeps making really REALLY terrible engages... constantly
At first: http://couchslobs.com/wp-content/uploads/2012/12/what-the-hell.gif
After a few more times: http://media.tumblr.com/6480a10fa670d11aeb34a51ac190f449/tumblr_inline_ml8p34xIub1qz4rgp.gif
When you're 35 minutes in and he keeps doing it: http://media.tumblr.com/57def8a72000cf3a50040d8aa9b12e4a/tumblr_inline_nemtdhNxz11sznexk.gif
When this results in the enemy team getting so fed that their tank has to be moved via forklift and he can nearly ace your team alone: http://media.tumblr.com/64e5d47f1656d181e4c43c29f52e1019/tumblr_inline_nej3tecDfc1s23mk9.gif
When said teammate starts complaining and asks why the enemy is so good: https://p.gr-assets.com/540x540/fit/hostedimages/1389284740/8015037.gif
That final, eleventh hour respawn as the enemy takes down your nexus turrets: http://media.tumblr.com/tumblr_lze01hxn8G1qfajwj.gif
After the match in the post-game lobby: http://media.tumblr.com/tumblr_m9e33hU6Oo1qgivl5.gif