I've been walked in on like 30 times
I always beat off to the interwebs, and my computer's always been situated facing the wall opposite of the door you come in through, and my mom would always do that knock-while-opening-the-door shit, and it'd give me just enough time to x out the pr0n and sit there with my dick out and lubed up hands obstructed by my body while she just stands in the doorway and tries having a full on conversation with me, what awful times
One day she did the knock while entering thing while I was just watching a skate video or something, and i flipped out on her, like "are you trying to see me jack off w/ that shit? knock and then wait for me to tell you to enter for fuck's sake"
Beating off is no joke to me.