Threads changing to already read status

I have noticed that when I come into the forums there are several threads that are set to new posts etc. I go into one of the threads and all of a sudden all the threads change to have read already (or whatever the proper term is). Anyone else experiencing this issue?

Backwards for me. Often already read threads go back to unread status. I don't know if it's a time/clock issue or a cookie thing or what, but it's weird.