1. Join a friends game via Steam
2. Friend stops the game
3. Click on "save" in the resulting "Server ended" dialog
During saving, Factorio crashed and showed an error: "Unexpected error occured".
Factorio saved the game successfully, the save game is not corrupt.
What did you expect to happen instead? It might be obvious to you, but do it anyway!
Factorio saves the game without a crash.
I can provide the resulting save game, if needed. It is however, quite big (~180MiB).