Spurious E13 message

I've been using vim for many years and have a strange problem that just cropped up.

I'm in vim happily editing, saving as I go and then suddenly I get "E13 File exists (add ! to override)" and I have to :w! to save the file. Then I'll be able to save the file a few more times, and then I get the E13 again.

I have only two buffers open; noreadonly is set; there's nothing I can see in :set or :setlocal that would indicate I have to "overwrite" an existing file.

No recent .vimrc changes that would seem to make a difference.

VIM - Vi IMproved 8.1 (2018 May 18, compiled Jun 3 2019 04:49:45)
macOS version
Included patches: 1-1450
Compiled by Homebrew

Any thoughts on what else to look for? Google yields nothing relevant except the old (2010) complaint on 73b which was posted here. It's driving me nuts.

thanks

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