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.
And it does happen to the best of us.
But luckily, Ghostcrawler commented today that this is something Blizzard is aware of and they're looking into ways to fix it.
I can hear the cheers from the heroic running crowd now.
Hopefully this is something they'll get out soon. A common suggestion is having a dialog box pop up asking "Are you sure you want to be saved to this instance?" or "This instance already has people saved to it, are you sure you want to continue?"
A solution like that would just be a matter of doing a search of raid IDs amongst the group entering and doing a few other simple comparisons. While I'm sure there are a ton of other considerations to make, the ease at which people seem to be able to come up with ideas to fix this problem is promising that a solution isn't too far off.