Yes, Tim, I've never had it happen on the 'same' Betaboyz Forum.
Ahh, it can be done, but you need to click the
right links - though that could just be me, see attached.
Granted, it's not so frequent on the BB site, but it's (mostly) producible by clicking successive title links of a post, most easily done on threads that have a dozen of more posts, see attached link for the one I'm referring to. It generally only takes about 5 or 6 different post link clicks for the 403 to occur, but they need to done relatively quickly. I never experienced the issue initially as probably a slow reader!
However, is it really a forum version issue? I've had some involvement in an old SMF site and having had a quick search through an old install, I can't see any page that looks like the 403 that we probably all see. Is it possibly something more webserver related that's meant to be blocking DoS attacks? Without wading through lots of posts, has anyone actually asked the zen folk if they know what it is?