Okay, Greg, that sounds rather worrisome.

You say the form is complex, so the issue is probably being triggered by
some code in the events, e.g. an implied save (e.g. find, filter, sort,
requery, assign RecordSource), something in Form_Timer, undo, writing
unbound controls to fields/bound ones, or another issue that involves timing
differences or interacting events.

Presumably you have split the database so that each user has their own copy
of the front end, so there is no interaction between users. If you have some
users on different versions of Access, it would be a good idea to use the
oldest version to decompile, and distribute an MDE. You also want to ensure
the users have the same version of msjet40.dll.

If your data is in attached SQL Server 2000 tables, you may have struck this
bug:
   http://support.microsoft.com/kb/903074/
The solution is to download SP2 for Office 2003.

There was a flaw in earlier versons of Access which we detailed here:
   http://allenbrowne.com/bug-04.html
If you believe it is this issue, you could verify it by writing the primary
key value of the record to a logging table in Form_Current and again in
Form_BeforeUpdate. If the numbers don't match, something has gone wrong
between those 2 events. I would be surprised to see this in A2003 unless:
a) you have many records loaded into the form (hundreds of thousands),
b) you have deleted record in that session before the bug is triggered.

The issue did resurface in Access 2000, as described here:
   http://support.microsoft.com/?id=238134
Interesting: that page was updated just 3 weeks ago.