It just happened to me with the "map" thread.
I figured out what the problem is. When a post happens to be the first post on the latest page in the thread (16th, 31st, etc.), the board doesn't calculate the page number correctly and sends you to the previous page of the thread.
I'm working on a complete redesign of the board software so I'm not going to fix this issue (along with a few others). I'm hoping to switch the board to the new software in a week or two.