One of the more annoying things that has come about in Wrath of the Lich King is the ease at which you can be saved to an instance. It used to be that you were saved to a heroic or raid instance whenever you killed a boss or got loot.
Now if you more or less step in the place, you end up being saved to it.
This can cause a problem if someone in your group forgot that they already ran the instance or raid during the lockout period (a day for heroic instances, a week for all current raids). That person loads an already completed version of the instance. The place has all the trash, all the quirks, except for the bosses. You'll spend a half hour clearing to the first boss, only to discover that he's not there – he's already been killed. And oops, you're saved to the instance too.
"Epic fail," as has been yelled out on my vent when this happens.