Aha!

So vB itself does not have the magic url feature? The code that adds that feature is something created by IB? Or does it come from a third party plug-in IB uses?

And what about the View First Unread link that appears on the thread pages? Is that a feature of vB itself or is it also added on? We have no evidence of malfunction for that code, do we?

I know from experience that debugging takes something like a minimum of 10x as long as anyone's most outrageous guess as to how long it ought to take, but I'm still puzzled as to why this problem is proving so hard to track down. We know the data is good, we know one piece of code consistently produces the right result for all browsers, we know another piece of code intermittently produces the wrong code for some browsers. Is that an accurate summary of what we know?