The bug

When trying to start a world from a very old version (such as 1.7) in a new version (such as 1.14.4), a warning appears, stating that you should make a backup of that world. However, that warning is grammatically incorrect, as it says "This world was last played in version unknown". It should rather say "This world was last played in an unknown version".

To reproduce

1. Create a new world in a very old version, such as 1.7.2.
2. Open that world in a newer version, such as 1.14.4.
   → The warning is grammatically incorrect