If a villager chooses to move out on the same say as their birthday, they will have their party, and you are unable to have someone else invite them to move in.
I found out a new one today but it isn’t fun. I thought voided villagers only roamed around on Main Street outside of the stores, nope, Monty was inside the museum WITH Wolfgang, same gallery room. I feel so guilty for not wanting certain villagers it’s awful being confronted with them.