Twice in my current campaign, there has been a point where players expected an easy win.
First was an undead encounter with a trap. They had the undeads fairly locked down, the mage had just dealt with most of the minions. The biggest undead was just about to be bloodied.
As it got bloodied, it did a close burst necrotic attack, seriously hurting both the paladin and a few other melees. Frightened that they had unlocked a new ability, the blew a daily or two on it, causing it to die - which releases another AE, killing the paladin and one other, causing the balance of the fight to tip. As it ended, 1 out of 7 was standing, and 2 of them never got up again.
The other fight that felt as a sure thing, was a demon. Again, the fight felt easy. So easy that the wizard (who had no magic missile) went out to scout, because there must surely be something else coming. Anyway, when it got bloodied, it got a round of extra attacks. But what changed this fight was it's ability to attack up to twice against targets that are bloodied. So as soon as a player got bloodied, he would drop increasingly fast. At the end, they won, but with 3 down, and 2 more (out of 6) quite close to dropping. Not exactly a walkover or what they had predicted.
Actually, come to think of it, the only times where it was easy to call a major encounter to end a specific way, and it proved right, has been when it has been quite clear (at least for me) that the monsters would wing. Nightscale for example, took way too many rounds to finish off the pesky players..
