These happen all the time IMC, but the one which ended last week's session was pretty stupid.
New campaign, only second month in, characters are 2nd level. Following track of nasty evil thing the local lord has asked them to track down and slay, enter room in which there is only one other door. A few feet in front of the door is a large insect-like creature (ankheg) half-in and half-out of its burrow. The ankheg is not moving, nor are its antennae. There is sufficient room between the ankheg's burrow and the door for characters to creep by if they're careful.
Characters decide not to creep past. They move into the room, as far away from the ankheg as possible, and ready missile fire. I ask each character to make a Move Silently check against the ankheg's Listen check. Thing is, the ankheg is well asleep and well fed, after having consumed large quantities of the organic mush in the room, and unless the PCs do something stupid, it ain't waking up. Cleric moves into the room, rolls a 3 on the Move Silently check - clang, clang, clang. Ankheg doesn't move. Cleric, thinking the thing is dead, jumps up and down and hollers. Now the ankheg wakes up. Readied missile fire bounces off the thing's tough hide (remember it has 50% cover) in the surprise round. It rolls bad initiative. Three of the five PCs in the room escape (the cleric, of Fharlanghn, casts expeditious retreat and is the first out the door!). The druid and the wizard are confronted by the ankheg, who misses its attack. The druid then casts a spell and runs, and the ankheg's attack of opportunity misses. The wizard runs. In the next round, the sixth party member, the only one not in the room until that time, enters the room to do battle with the ankheg and protect his friends - the mean, chain-wielding fighter has only 5 hit points at this time, though. He misses. Everyone else keeps running. The ankheg attacks the fighter, rolling a 19 (d'oh, no critical). Thing is, an ankheg bite does 2d6+7 plus 1d4 acid points of damage. The roll - 5,6,4, plus 7, for 22 points of damage. Fighter is neatly cut in half, and the nearby druid is covered in his acid-burnt entrails. Nobody much speaks to the cleric after that.
Cheers, Al'Kelhar