>>.. Sometimes, when a user has two forms open, one of the forms won't give up focus.
>Nonetheless, I think I need to test this with Event Tracking on again to see exactly what's firing.

Tamar, if event tracking doesn't help there is always "Binary Brute Force" ™

Remove half the event code.
If it still fails, remove half again.
etc.

Obviously, use a copy of the project, and save the halves as you go.
And easier by opening the form as a dbf, so you
can see all the Valid(), Losfocus(), When() and other suspicious and shady looking methods.

The few times I had issues that could not be found using the debugger, this did work.
It took a lot less time than I feared.

Good Luck!