Yeah, modified a guy I know's "family" computer once back in high school in retaliation for trying to mess with my stuff, replacing the windows bootup splash screen .avi with some hardcore he had hidden on the HDD from his mother.
Kinda backfired, in that his little sister (12, iirc, at the time) turned it on first the next morning as their mother walked downstairs with her first cup of saturday coffee, instead of his mother.
They had to repaint the wall where she spewed coffee from across the room. They also blamed the guy, and reformatted the HDD to get rid of his porn. They never believed him when he said I did it. I was the "good" friend, after all.
He never messed with my crap again afterward, though.