Sometimes it is just as rude to arrive too early as it is too late, like when your host isn't dressed yet. Sam is almost more embarrassed than Penny when he walks in on her naked but she is definitely more annoyed. Sam covers his eyes and turns around to stammer out apologies but that just makes it easier for Penny to get a grip on his tighty-whities. Since he got to check out her ass, she is going to wedge his undies in his ass. Maybe getting wrecked by wedgies will help him remember to knock next time.