When I start a new town I go into my library and pull familiar Sims and just plop them into spots around town. I don't intend to really play them but every once in a while I do a switch active household and check on them. I like to see what they might have done on their own. To my surprise I've found two of them have had invisible babies. The moodlet hangs over the crib,the parents pick up the babies and interact with them,they are just invisible.
Just curious to see if this has happened to anyone else.