It could be something to do with the browser cache, when it gets full it could start misbehaving.
Its not just on this forum that Ive seen people say that read posts go a bit funny on the odd occasion and clearing the cache/temp files normally does the trick
(God I feel like Ive just used the typical AOL/BT help line get out
)
Seriously though both browsers can seem to flip on occasions with forum posts regardless of the type of forum software/forums.
Recommended things to try are:
~ Log out and back in again.
~ Clear temp internet/cache files