I have to admit that until about 10 minutes ago I thought I had flock under Win32 completely under control.

Ditto :-)

Here's a bit of wild speculation. Perhaps the shell tries to get a lock on any file it opens for writing. If the (non-blocking) lock request is refused then it gives the error you see. If it were Linux, strace would confirm or deny that theory in a minute but of course it's not :-(

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