Or maybe you've switched the calling code to use the new 100% compatible and much faster Order_XS module. Yet here's module X, still pulling in the now-unused pure-Perl Order module. It's code it doesn't need, but everything keeps working, so nobody cares (or even notices).

A couple years later you're installing the app on a new server, and the sysadmin asks, "do I need to install this Order module?" And you say "no I switched to Order_XS years ago. Don't bother." And then...