When your sim visits someone else's house, can they not use everything in the house like they're at home? Example: I sent my pregnant sim across the street to meet the neighbors, and she had to pee real bad while she was there. She chatted with a couple of them for a minute, then went to use their bathroom. Instead of using the toilet, she proceeded to pee all over herself while standing in front of the toilet. Multiple times. She continuously peed herself for a good five RL minutes before I finally managed to get her to run back to her own house, leaving the neighbor's bathroom flooded with piss.
I would have been annoyed, but I rather enjoyed watching her distress.
I'm thinking maybe it's because she didn't know the neighbors well enough? I mean, I wasn't trying to have her eat out of their fridge, she just had to pee.