Scenario: case sensitive volume.
This is not rare.
For example, all 32 TB of photos in my area are on case sensitive drives because most web servers are case-sensitive and the photos are ultimately stored on servers using six-eight characters from a 41-character mixed case set of possible characters which has been chosen to eliminate profanity but still basically use most upper and lowercase letters and digits 1-9.
Steps to recreate:
1. Edit a photo like ___1234.JPG with an external tool.
2. Save edited copy of photo in same folder with a name like ____1234_Edit.jpg
3. Add a crop to the photo ____1234_Edit.jpg [from within Photo Mechanic].
4. Then do Command+S to save the crop in the original location as a separate file.
Issue:
Normally the file is saved (and should be) as ____1234_EditA.jpg. However, the original edited photo file is overwritten.
Possible diagnosis:
It seems like a capital extension is used to check for existence since that is the default output preference in PM, but a lowercase extension is used to write the output file since the origin file had a lowercase extension.
I haven't noticed this up until now because all editing tools used capital extensions as did the original files, but a new editing tool has only lowercase and no way to change it.
I realized it was happening and recreated it only after losing about 50 photos edited with this specific tool in the last two days, which were overwritten by crops of them made by Photo Mechanic after doing the editing.
[Changes: added clarification to step three that the crop was added to the edited photo from within photo mechanic, not within the external editing tool that already produced the original edited photo.]