I suddenly appear, and make this song play in the heads of all enemies!
I then disappear while saying "I WAS ONLY HERE TO DO THAT!!"
Some games try to avoid this, but it's becoming a bit of a problem.
"Oh, that Mega Armor dude made you run out of lives, well if you want to restart, you have to give us $10 just to get another life and keep your saves!"
It's even worse when there's that one weapon that everyone else can buy, but those who are a bit poorer has to play their whole lifetime and go 1,000 games straight as MVP with no deaths and they have to kill all the Gods of the Killyourealm. No, just, no.
EDIT: OHAY, I'M ACTUALLY POSTING/EDITING!
I know this is pointless, but I hate it when "F2P" games do this.
"Ohai, you completed the tutorial and first levels of the game! Great job! Now give us 20$ so you can play the rest of the game!"
Seriously, don't do this. You might as well make your whole game 20$ so that people don't feel like you're mugging them.