This is an unhelpful post, because I don't have enough details about what happened.
There's a follow achievement that I've never done, so I thought I'd try to follow someone. Someone I don't know, so it felt a little creepy. And they were just standing there at one point, and I was, like, "okay, time to give up on this", so I clicked a signpost instead. And boom! I was in someone's house.
I should have paid attention to where I was and where the house was, but I did not. My house is in the Tii beehives, so I was near there, but I was not yet on Tii street. (I knew it was buggy behavior, but I wasn't entirely sure that it wasn't my very own house at first; I thought the lag meant that the items strewn all over the floor in my house weren't showing up.)
I nibbled their piggies (seemed harmless) and repaired their broken tinkertool (which I hope was helpful) while I was there and then left, returning to the street I had come from. Which, when I think about it, was probably a street immediately adjacent to Tii.