I've noticed the last patch brought with it some server stability issues.
I ran some heroics with the guild the other night and sometimes we'd get kicked out to the character selection screen. Upon logging back in, we'd be back at the start of the dungeon, with the dungeon having been reset. And we'd no longer be grouped.
Not a big deal when it's a guild run, or even a same-server run, as you can just invite people back in. However, in a cross-server PUG, you can't invite everyone back in, so people are essentially kicked from the group.
Pretty annoying if you have to then sign up for another dungeon and wait the full queue again. Imagine waiting 20 minutes in the queue, join a dungeon, clear everything up to the last boss, get booted and then having to queue up again.
Waste of time much?