Losing a Baseball game
Having the football pulled away by Lucy
In an embarrasing situation
Is used also by other members of the peanuts gang and can be used as a roaring sound to scare someone.
Linus: I told her about how you're madly in love with her,
Charlie Brown: Aaugh!!
Snoopy: It's not the things that go 'bump' in the night that scare me. It's the things that go 'AAUGH!'