I think it was the flirtatious joke that did it, too. I'm having a hard time trying to replicate the exact issue though. I don't know if it makes a difference but every time it's happened to me, I'd been playing that same household for a long period of time. In the new tester household I made specifically for trying to get it to happen, I noticed that everyone in the group will react to things you're doing to one specific sim as if you're doing it to them but the actual romance bar popping up isn't happening. For instance, if I got the tester sim to romance her husband, everyone will be embarrassed/"awkward conversation", and a few will become flirty. I'll have to play with that household for longer... maybe my individual saves just got corrupted somehow?
It's kinda frustrating because it's hard to force it to happen, but the times it has happened have been really annoying. You can't even choose who your sim is talking to in group conversations and a lot of the time, they just start talking with anyone and everyone in the room. I think the actual issue might tie into another issue where flirty sims in general are extremely aggressive, because I think all the sims who ended up with the romance bars were in flirty moods when it happened.
I haven't owned the game that long, so it's not like the older households would have had the same issue that Crin mentioned that got fixed. I'm going to keep trying to get it to happen in a fresh save, and if I don't, then maybe it's an issue with my save games. Overall, not the end of the world, just an irritating bug that I've encountered.
