This is a maligned error message seen by players on realms during peak instance usage that prevents a character from entering an instance. This instance cap was implemented by Blizzard to address problems seen when the number of instances active grew too large and started affecting the experience of players already inside instances. Blizzard has been working on a solution and started implementing a solution on most U.S. servers. Some positive reports have come back, but not in a widespread fashion or on known high population servers.