I feel like this happens whenever I upgrade wordpress or edit themes, at distant enough intervals that I forget what the problem was last time I fixed it.
So, I have a long post and want to use <!--nextpage--> to split it up.
The problem; the whole post displays. The page links are still present at the base of the post, though clicking on the links doesn't do much. How do I get these stupid tags to behave appropriately again?