Here's something else I'm noticing. When I bring up the forum main page - the first page I am looking at - if I click on the back arrow and hold it, a list of previous pages comes up, and it will show one line for bb.com (the main page I am viewing currently). Now if I next go to the Garage Sale, when I click on the back arrow, it will display the current page (Garage Sale) and under that will be at least three more Garage sales pages (even though I have only opened the one page and did not open anything on that page). So the extra pages displayed explains why when I click the back button - it probably is working but it has to display the other three (same) pages before I hit the actual previous page.
Hope this is understandable and gives some insight into the problem.
This sounds like it may be something related to either your server, the network, or your user's computer system.
If he is clicking multiple times and many different windows are popping up, he may have something slowing down his computer, a bad browser plug-in, a virus, or just simply a sticky key (SHIFT, CTRL ...etc)
If his network is slow, the page could be delayed from sending/receiving data from your server to his network.
If this is happening for more than one user, and since he mentioned that it's happening at his home and work (same computer laptop?), it could be an issue related to your server caching data, and being slow about it.
Is he on an apple product? I am aware that older versions of Apple iOS (iPhone and iPad) sometimes required multiple taps on navigation elements, where MacOS/Windows/Everyone else acted correctly.
If he is being given an ad from one of your pages, it may be slowing his browser down -- does he have Adobe Flash installed? Thats often the cause of much of this type of shenanigans.
It really could be anything. But there is is nothing in the stock UBB.threads software which would cause this behavior of his browser.
Can he reproduce it on UBBCentral.com, or UBBDev.com ?