I finished up a long Uni run with 8 out of 11 kids, so it had been a long time since I played my prime legacy family.
My heiress graduated, so I moved her back into the legacy house. No messages warning about too many Sims appeared.
When I opened the lot, I realized the mistake. The lot originally had 2 adult males, 2 adult females (both pregnant) and 2 toddlers (twins) = 8 Sims. Post move the lot had 2 adult males, 3 adult females (2 of them pregnant) and ONE toddler. The second toddler was nowhere to be seen.
The grandfather was carrying out the process of preparing smart milk for the vanished toddler. He was holding his arms the way the Sims do when they're carrying a toddler. He went over to the smart milk machine, made the smart milk and put it on the floor.
The toddler's mother had memories of giving birth to "Fir" and "$Subject." (Fir is the existing toddler.)
The Tombstone of L&D didn't show the vanished toddler as a missing family member. I didn't see anything under the lot debugger except "Force errors," and I wanted to hold off on trying that.
The twins were the product of cheesecake, so I'm wondering if that contributed to the glitch.
I can't find the missing toddler in SimPe, so I think she's toast, but I'd welcome comments. This generation has been plagued with weirdness!