In my case I have the default posts per page set to 20. If a user sets this to 40, for example, the posts load initially, but then disappear after 5 seconds. More details here: Auto Thread Refresh Issue - losing posts
I guess a couple of our members posted the same issue today:
I seem to be having either an issue, or can't quite figure out what is happening when I view some threads. When I open some threads, I get to view everything for what appears to be the 5 seconds before automatically "refreshing" and removing several of the first entries of the thread from view. What I mean is that if previously the thread replies #1-23 were visible... after the refresh, #4 is now the top reply and #4-23 are visible and I see no way of being able to read #1-3.