I sometimes put toilet paper over the seat but then leave it there for the next person.
I have logged into someone else's Webmail at school inadvertently and then proceeded to read an email that looked interesting.
Sometimes I pretend like I'm in a competition with Steve Horvath over who has more Collegehumor updates. But he doesn't know we're competing so it's really just me looking to see who has written more. As of today, 50-49, me.
I have intentionally taken longer than necessary to respond to an IM so that the person I was talking to thought I was busy talking to other people.
I sometimes order a Diet Coke at a bar when I don't feel like drinking, and later tell my friends its a rum and coke. When they taste it I agree that it's really watered down.