A tank was considered uncrushable when they were immune to crushing blows delivered by raid bosses. In order to do this, the attacking mob's attack table must have been filled completely with other attack results that take precedence over Crushing Blows. Against an opponent 3 levels higher than the tank — Boss mobs are treated as being 3 levels higher than their targets — all combined avoidance need to add up to 102.4%. However, changes in Wrath of the Lich King made this largely a non-factor for raids. It still applies for mobs 4 levels or higher above a player character.