...and proceed to utterly wreck the level 11 party.
Myru the Slayer, Isaac the Inquisitor, Ada the Sorcerer, and Xavier the Bard have just been given a quest to travel to a nearby abandoned temple to retrieve an artifact. After a 12-hour journey on camelback across the desert, they arrive and are greeted in the first room by a level-appropriate encounter. It proves to be somewhat difficult but survivable; they retrieve some treasure and heal up before the next room.
Here's where things take a turn. We players were thrilled to roll nothing above 20 on the random monster table used for this particular dungeon, and so we are greeted in the second room by a walrus, a bear, and an elephant, all completely ordinary - and all CR 7 against us level 11s. We figure it to be an easy fight and don't even start buffing ourselves up before stepping in.
Contrary to our expectations, we all roll incredibly low on our Initiative checks, and the animals all roll very high. (This is to become a pattern this fight.) In the first round, the walrus rushes over to us, clustered just outside the doorway, and tramples over us all, knocking us prone and doing some decent damage. The elephant comes over and stomps on Isaac, while the bear moves over and prepares to attack whoever gets up first. Now we are stuck between the animals with nowhere to go. Isaac gets up and gets grabbed by the bear's maw, immobilizing him; Myru tumbles around to the other side of the elephant hoping to get some flanking; Ada casts a lightning bolt at the elephant, doing near-minimum damage; and Xavier starts inspiring courage while laying on the ground.
The second round progresses in a rather similar fashion to the first. The walrus tramples the three of us still stuck outside the room again; the elephant moves back and tramples Myru to the ground; and the bear starts eating Isaac, keeping him grabbed so that Myru's flanking hopes are in vain. The bear falls soon after, but for the next few rounds the walrus and elephant continually trample us, keeping us prone on the ground and unable to fight at our best.
It must be noted here that the DM has rolled, out in the open, 17 or higher on 90% of his d20 rolls.
Eventually Isaac stands long enough to slay the treacherous walrus, but it is too late - Myru, faced with this massive elephant to fight all alone, has been trampled into a small pile of gore on the ground. Isaac and Ada finish off the elephant in retribution for Myru's tragic and terrible death. Having lost a key party member in the second room and faced with the prospect of going through the rest of the dangerous dungeon down a man, the party abandons the quest and retreats back to the city from whence they came.
And that, children, is why you never judge an encounter by its CR.