Why it HappensEdit
This happens if The Sims 3 or The Sims Medieval fails to load the Saved game. It might happen if The file is corrupted. The Text that appears is this: "A serious error has occurred while loading "name". It is strongly recommended that you restart the application." (Replace "name" with the Town name that game failed to load. (Example: NaN.sims3 or NaN.tsm))
It can also happen when starting the town for the first time, and the game fails to load the town. In the Sims 3, if the file has too much characters, it might fail to load. If it does load, you can play, but if the game tries to save, it will have a Save Game Error.