World of Warcraft has an undocumented limit of five instances per account per hour to prevent players from overloading the game's instance servers. Under normal circumstances, players will not be in danger of hitting the limit, since it's relatively hard to chain any useful dungeons more rapidly than 12 minutes each. The Cataclysm elemental invasion makes it much easier to hit the limit, as it is possible to queue, enter and clear the instance, and requeue inside of five minutes.
The automated dungeon finder disregards the five instance rule (which is ACCOUNT-wide, not character specific), so you can easily run vastly over the limit without any idea that you've done something wrong. Then your guildies ask if anyone else wants to join the raid, you try to join in, and you're not allowed to zone in until the timer wears off.
It's not the largest of issues, but it's pretty disappointing for you (and potentially your group) if it happens unexpectedly. There's no UI that informs you of how long you have to wait, so you're just left to guess how long it will be before your fifth-most-recent instance hits an hour old. It's a bit disappointing that Blizzard did not take this restriction into account when designing the world event.