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.