Steps to reproduce:
Open emacs -Q
M-: (progn (linum-mode) (hs-mode))
Insert a block of comments
M-x hs-hide-block

This might be happening because (window-end nil t) does not return an
updated value after the creation of a "hiding" overlay. Although I haven't
explored it deep enough, to be sure that it is indeed so.

Evgeni