Perhaps the nastiest thing I ever did to players was a group of ogre rogues.  I'm pretty sure the PCs would have won<em>eventually</em>, but my co-DM convinced me to fudge a couple rolls early on so that the first PC to charge didn't simply get slaughtered by flanking ogre sneak attacks.<br/><br/>Oh well.  It was still funny<em>and</em>entertaining, and that's what counts.