If viewing as a guest, not as a logged-in member...
Clicking on a "Go to Last Post" button in the forum index leads to an endless loop of page reloads if the last post is recent. ("Recent" varies; I've seen it happen with posts that are between one minute and 20 minutes old.)
In such cases, if you click on the thread title and manually navigate to the end of the thread, the Last Post listed in the index isn't there.
Occurs in Firefox and IE, with or without caches flushed before loading the site.
This first appeared after the format change last year (apologies MM, I just now got around to reporting it).