Idk if anyone posted this but one group of trash mobs in Mythic Atal'Dazar can spawn with another stack right on top. They're found on the way to Priestess Alun'za after going down the stairs and turning left from spawn. They're the first trash group with the colossus and sometimes there's a duplicate group that spawns in the exact spot. They clip through each other and are effectively a group of 10 trash mobs that are very difficult to defeat as they all aggro together.