When you finish an exam early, but still have ages to wait until it ends. With nothing to pass the time, your mind wanders to porn/ the hot girl in front of you and you get and embarrassing erection.
Guy 1: oh dude, that exam was so long!
Guy 2: I know, I was so bored
Guy 1: hey, weren't you behind that girl Jessica?
Guy 2: yeah, I got a massive exam boner
Guy 1: wow, embarrassing!