My most recent was print printf("%03d\n", $number); and it took way to long to figure out where the extra one was coming from.

And another one from a long program that wrote a bunch of files, but got rid of any empty ones.

# The code as it was executed
open filehandles
print "to file"; # This was anything from
                 # a few to huge # of items
#cleanup dir
go through directory and unlink any empty files
close filehandles
[download]

For some reason if the program didn't have much to say I'd wind up with fewer files than expected.