There's a level of Hell reserved for
- people who don't write tests
- people who remove a failing test, rather than fix their code so the test passes - because the test "must be wrong"
- people who don't reduce failing code to a minimal test case
- bug reports that just say "it does not work"
- people who roll their own version of X because it will be more efficient
- before finding out whether more efficiency is needed
- and write something less efficient than X anyway
- people who don't use CPAN