I think that there is a bug in the site whereby if a post is deleted it doesn't detract from the total number of posts in the thread. It's also possible that there is another unrelated but that is causing the number of posts as counted and the number of actual posts to be in disagreement such that one is incorrect. The net result is that going to the last post in the thread either by using the end key or by clicking the pop-up that takes you to the last post in the thread instead takes users to a blank page.
If I manually enter the post number I want to skip to in the url I can find the actual last post based on the last post number minus one but it still not the correct number for the last post since the post count in the thread is apparently wrong. If I dial the post number back far enough in the url (by about 4) it will eventually catch it and dial it back to the actual last non-deleted post number which appears to be the post count minus the number of deleted posts minus half that number again.
My testing has been done in Chrome.