I have no idea why, but the order of the output of the permute() function in the Perl library Math::Combinatorics depends on the number of elements in @ARGV. This resulted in a rather frustrating debugging session, in which it was assumed that the output was deterministic, and one could reliably shift off the null permutation.

Seriously, when the mere act of adding a command-line option that is completely ignored by the program changes the output, your library is kinda fucked.