I'm forwarding this bug report verbatim from http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=411033 as I think A. Costa has explained it better than I could have:
I had the bad luck to run 'pngcrush' in a
directory with only two files:
% ls -log total 32554 -rw-r----- 1 16638693 Jun 14 2005 Jettaradio0001.png -rw-r----- 1 16562597 Jun 14 2005 Jettaradio0002.png % pngcrush * [...] Recompressing Jettaradio0001.png [...] Best pngcrush method = 9 (fm 5 zl 2 zs 2) for Jettaradio0002.png [...]
If the above isn't completely clear, 'pngcrush' examined the first file,
then overwrote the second with an optimized compression of the first.
Result: two files with different names and the same image data, while the
original second file's image data was deleted.
It would be better if 'pngcrush', when commanded to overwrite a file,
did either of two things:
1) Prompt the user: "are you sure you want to erase fubar.png?" 2) Quit, with an error message: "warning output file exists", and a "--force" switch could override that.