Random Townies keep walking in my house. My Sim doesn't even know most of them, but all of a sudden, they're upstairs on the computer, watching TV, and walking through my home. It has been happening ...
Ideas
I've had this happen as well although it was a long time ago. The worst/funniest was my sim entering his bathroom only to find a stranger taking a bath. As there is no way to ask a sim "what the [strong word of your choice] are you doing in my bathtub?" or tell a sim to "get out of my house you rude naked man!" the only option my sim had was a friendly introduction.
I'm not sure about the rest of you, but I can't say "Hi, nice to meet you, what's your name?" would be my first reaction to a random naked man in my bathroom.
In general, wish there was a way to tell offending sims to get out, like they can if your sim randomly start cooking at their house. It doesn't really seem fair that they can throw you out, but you can't throw them out...