Most modes that use custom fontification (ex occur-mode), won't turn the font-lock-mode off. So hi-lock would add a keyword, and remove all existing fontification. The below patch fixes this behaviour.
Evgeni
Modified lisp/hi-lock.el
diff --git a/lisp/hi-lock.el b/lisp/hi-lock.el
index e2dc4ea..e16ed76 100644
--- a/lisp/hi-lock.el
+++ b/lisp/hi-lock.el
@@ -689,7 +689,7 @@ Otherwise, read face name from minibuffer with completion and history."
;; Refuse to highlight a text that is already highlighted.
(unless (assoc regexp hi-lock-interactive-patterns)
(push pattern hi-lock-interactive-patterns)
- (if font-lock-mode
+ (if (and font-lock-mode (font-lock-specified-p major-mode))
(progn
(font-lock-add-keywords nil (list pattern) t)
(font-lock-fontify-buffer))