darcs init --repo R
cd R
touch foo
darcs add foo
rm foo
darcs wh -> "No changes!"
touch foo
darcs add foo 

the final add errors with
"The following file is already in the repository;
note that to ensure portability we don't allow
files that differ only in case. Use --case-ok to override this:
...
darcs failed: No files were added"

However, darcs wh shows the file as being added.

This probably isn't that likely to occur, but it'd be a slightly strange
error for a user.