There’s a possibility that your save game becomes corrupted. The error 12 an “out of memory” error and most often happens with large save games, meaning neighborhoods you’ve been playing for several generations.
To avoid this is in the future, here is a small piece of advice:
- Copy your saves folder to another location periodically. Depending on how often you play, do this every week/2weeks/month.
- Always keep two copies of your save game. Instead of hitting the Save button, choose Save as every time. Save your town to sunsetvalley1 today, and sunsetvalley2 tomorrow alternating between your save games. That way you never loose much progress if one of your saves becomes corrupted.