We've long (1-2 yrs) had a situation in MOSS2007 whereby "phantom" rows get
created as a result of a custom edit form whose records are filtered by
current user. It seems to be associated with the tiny arrow icon that takes
you to the next record (if any). We never solved the underlying problem but
made it go away by building our own form from scratch. We had one of the
"known, named" gurus look at it, and he said he guessed that somehow the
list's default edit form was getting called/triggered.

Interesting. That gives me an idea. We have multiple custom edit forms for
the parent list and we're not using content types -- I know, I know -- and
we had some "switching" trouble when building the original forms. I wonder
if a band-aid fix would be to make one or more fields required (right now we
do custom validation based on the edit form because the requirements vary).
That would keep additional records from being created, even if it didn't
nail the problem at the source.

I can't speak to exactly why you are seeing this issue though we have seen the
same thing before. As you have already stated it was because we were messing
with the default new/edit/disp forms.

We also learned the hard way what content types were good for.