A really embarrassing one for a geek like me was in Phantasmagoria 2. At one point you need to check your e-mails, but can't get to your desk. The only computer you can get to is the one belonging to your boss.
That had me chewing the carpet for WEEKS, and eventually I was forced to look at a walkthrough. Duuuhhh...
Spoiler:You hit "Log Out" on your boss' computer and then log in with your own name and password.
