[]What's strange is there are some server/setups this works flawlessly on. I talk with some people that never have a problem or complaint about it, while others have all sorts of problems. [/]
I run our boards from multiple servers behind a load balancer and, before the upgrade to 6.5.5 (from 6.4.4 I think), I had straight, round-robin balancing; meaning anyone who hit a page had, pretty much, an equal change of hitting one of the 4 servers behind the load balancer and each user would hop around between servers the whole time they were looking at the boards. When I did the upgrade, we started having the same problem mentioned here and in a couple other threads.
It looks like, during a user's visit, posts that they have read during that visit are stored in a PHP session, which is local to a particular server. So, when a user would bounce between our 4 servers, they'd get all sorts of weird results because each server behind the balancer had a different idea of who had been where. All I had to do to fix it was turn on 'persistence,' which basically means that a user, over a period of time (usually a single visit via a temporary cookie), keeps hitting the same server over and over again instead of bouncing around all over the place.
I bet most people in a hosted environments don't even know if they're site is being served from one host or from multiple hosts behind a balancer and, if so, whether that balancer has a notion of session persistence or not.
