Of whatever screen we're looking at, I'd say at bare minimum a good third of the visual information it displays in an hour is not something we want others seeing. Whether that's Thundercats reruns, saucy e-mails or online personals, or certain game sequences that, out-of-context, might make it look like you're up to something else.
GamesRadar, I know I always toss out the lists they're working on, but they have a list of game moments you don't want your family or friends walking in on. Super violence is obvious, especially if you're in a family dwelling. So are strip clubs in Grand Theft Auto and Carla and Lucas having sex in Fahrenheit. But GR also cites "the whole thing" from World of Warcraft -- as in, caught playing that is no different than getting busted with porn on the screen.
Age and context matter, I think. If anyone caught me playing We Cheer or Ultimate Band I might be in for an uncomfortable intervention, too. So, have you ever been walked in on at an awkward moment in a game?