Prevent "AHAH the Right Way" in Drupal from Breaking with Validation

So there has been a lot of blogging and documentation in the past about the "right" way to do AHAH in Drupal. I think these people make excellent points, provide good documentation, and are pushing these types of dynamic interactions in Drupal in the right direction. I think a lot what's out there about this "right way" to do AHAH in Drupal misses some critical issues, though. Maybe someone else has blogged about it already, but I'll share one pitfall that I learned to avoid while doing AHAH.