The page itself is a huge file. 5 tabs, a couple with subsections, and several options lists filled from a database. So I don't think it would be proper to post it. Maybe I can dig out a small subset that'll be enough.
But, I don't know that it has anything to do with being hidden. It seems to be a problem with clicking on a text box.
One tab is contact information, and it's where I notice the problem most often.
Column 1 -
Option list (changing this calls JS to change the address fields that follow - ex: Home, Work, etc)
Address line 1 
Address line 2 
State  (Drop down option list)
Column 2 -
Phone description option list (same as address)
Phone Number 
Email description (same as address)
I have some JS code that uses a XMLHttpRequest object to fetch the city and state based on zipcode, so I often click on the zip code field. But, after typing in the zip code, tabbing out takes focus to the first link on the page which is not in the tab area. It'll do the same with the JS not active, so I don't think that's the problem. Same thing happens if I click on the address line 1 field, enter the data, and tab.
Is it possible that onBlur causes problems? Each of the fields updates a hidden field upon onBlur. But, the problem only occurs when I click in the field, not when I tab into it.
Edit - One thing. After changing tabs, nothing shows as active which I assume means that the active field is hidden.