Just got kicked out it again after being logged in for less than an hour. (Safari 5.0) I think I hit "show unread posts" and ended up logged out.
Also had my "show unread posts since last visit" go from being pages to being ~10 threads between getting logged out and then logging back in moments later.
Glowdart, the latter happens to me quite regularly and has done so forever (particularly when I try and trick LeechBlock by switching to Safari). Interestingly, the "Show Unread Replies" tab always keeps all the threads.
How odd ... It's only been happening to me this last month or so, but I'm also not running any add-ons. I also haven't been having the other weird issues with posting and quoting that other people mentioned, so for a brief shining period, I thought, "Wow! For once, the weird web glitches are not because the program is overly-PC friendly! How often does that happen?"
(And, it's kept me logged in for hours now since getting booted this morning, so....??? Gremlins?)