When opening a new world in a new version, you have an option to "backup and load."

If you click it, the world will attempt to back itself up. However if it fails, the world loads anyway without pause, potentially corrupting it and leaving you with no backup.

Janky steps to reproduce

1. Create a world in 1.15
2. Copy it so you can test multiple times
3. Create an excel document in the world folder
4. Open the document in excel and save some stuff, keep excel open
5. Open the world in 1.16
6. Click backup and load