comm is a really useful tool, with one big caveat — you must make sure your input files are all sorted the exact same way. If not, you can get unexpected results, and worse, might not even realize it.

This may seem obvious, but there are many tiny ways that sorts can differ between locales, operating systems and programs (e.g. Excel), especially when dealing with Unicode. It may look the same 99% of the time, and you may not realize until later that you’ve accidentally filtered out values.