Last night I put my players up against a pack of displacer beasts and it was...really boring. None of them had any special detection powers, so it just meant the combat dragged on for twice as long as normal. And since the displacer beasts don't have any other special abilities to make the combat interesting, they performed basically like a pack of ordinary animals with double hit points.

Not long before that I put an epic party up against eight different monsters, and it also became glacially slow but for the opposite reason: there were so many full attack routines with four or more attacks, plus swift actions and long lists of special abilities that it took over an hour to play a single round of combat.

What encounters have you tried that didn't work?