Joe, you have been at this a lot longer than me but I have yet to come across a Mudcat system failure on the registration (not that I'm doubting they happen). The most common ones I can think of are:

Cookie problems as you have mentioned.

Mistaken Usernames, e.g. there could be a Jon F and a JonF, someone may come back to rejoin (reset cookie), see JonF and assume that is them when really JonF is someone else and the person trying to rejoin was registered as squiggle, etc.

Missunderstanding of the difference between regitering a new member and re-setting a cookie. Maybe the user interface could be re-designed to make this more clear.

Confusion caused by missleading and inconsistent terms within the registration system. I know that you have mentioned this one to Max and hopefully it will be sorted. I can certainly see why a user may keep trying to choose passwords to resolve "CodeName" in use.

Jon