Assume you have a direct link to a post, viewtopic.php?p=10#p10 and you have 10 posts per page in the settings.
If post 10 and 11 in that topic are posted the same second (rare, I know, but has happened), then the above direct link would take you to page 2 instead of page 1 where the post is.

This is because the determining of the right page counts per post time. The fix would be to also check the post id. Only counting per post id would also work but might give the wrong result if someone is messing with the database and changing post time. IMO checking for both is the best solution.

You'll get a pull request as soon as I have learned how to do that.