The following scenario is a big difficult with our versioning code:

1. Save file in new version of Blender.
2. Load and save the same file in an older version of Blender.
3. Load the file again in the new version.

Not all our versioning code is idempotent and it can lead to hard to detect bugs when some versioning code runs twice. Showing a warning when saving a .blend file in an older version of Blender might help mitigate that issue a bit.

E.g. T93252.