It looks like for awhile Harry Johnson might've became a regular character, and all of a sudden he just vanished (like Adam). I've only watched up to DVD set 16, so I don't know if he comes back. It is no big deal, but they must've wrote off the character.

Yes, Harry Johnson does not appear again after episode #669. Craig Slocum had not shown up for work on that episode (Edward Marshall filled in for him), which might be part of the reason for the character's sudden disappearance.