There was a bug under Mac OS 9 (and maybe Mac OS 8 too?) that, under a special circumstance, prevented some files to be locked, unlocked or even deleted. They would sit there in your trash forever and prevent you to empty it even when the OPTION KEY is held down. Rebuilding the desktop does NOT resolve the issue.

Trying to get file info with ResEdit results in the following error:

The reason this is happening is generally due to the file type/creator code not being set properly or at all, which highly confuses the Finder. These files usually come from the internet (such as .iso files) and are generally transfered using a FAT32 (DOS) or NTFS (Windows) partition which does not store any resource fork (so no file type/creator code) to begin with and increases the chances of this happening.

To resolve this issue:

1) You need to use a file type/creator changer app, such as: Creator Changer 2.8.4

2) Launch your type/creator code changer app and select the problematic file. If the file is locked, it will probably complain it will not save changes, but guess what: It actually does and it will work, so proceed.

3) You will notice that both type and creator boxes are currently empty, which is not supposed to be with any valid file on a Macintosh volume. This is how you will fix the issue: Type '????' (4 interrogation marks) in each of the 2 boxes like the following and apply the changes (click OK).

Now, your file will have a default file type/creator code and it will be seen as valid by the Finder, allowing you to lock, unlock and delete the file. Moreover, you will now be able to get file info with ResEdit and use/edit the file's resource fork as you please!

Cheers! :D