MichaelStroke:I'm not even 100% sure that it affects your completion percentage if you don't actually forfeit. I imagine if you got disconnected any of 3 times it would, but if the opponent is the one who doesn't come back for 150 seconds or actually clicks "forfeit", it may just go to him.
Nope - either is affected. The tag is named "completion" not "quit", and it gives a +1 only at the finishing line. Me thinks that this line is at the "Main Menu" in the game window.
This may be the reason that A/S were excluded after a short time: One player discon/quitting would drag down three others.