App says I don't have permission; Finder says I do

TidBITS Talk
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I tried to save a document and got this error message: "The document could not be autosaved. You don’t have permission to write to the folder that the file is in.”

Finder says I have read and write permissions to both the folder and the file. Any hints?

In case it matters, the application is Preview, the folder is a subfolder in my Documents folder, I have edited and saved the file many times in the past year (with the same name and in the same location), I have not had any recent OS updates or other changes. (I’ve been out of town without the computer for over a month, if that matters.) I’m running macOS Sequoia 15.5 on a MacBook Air M3. Thanks for any help.

Edit to add: I just saw that Preview says the file is Locked, and I don’t have permission to unlock it.

Edit to add more: I created a copy of the new file in Documents, closed both the old and new, moved the new file to the subfolder (overwriting the old file), and opened the new file in the old location. Maybe everything works now. I’m still wondering what happened.

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You might find this useful;

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Thank you (seriously) for that rabbit hole. Howard mentions stuff that meant something to me 30 years ago; some of it I can remember. Fortunately, the problem seems to be solved (as mentioned in the second edit of the original post); unfortunately, that fix almost certainly destroyed any forensic evidence (if any). My guess is that some cosmic ray set the Locked bit, but that shouldn’t have happened. Oh, well, time to move on.