The first spawn in the first area of Hlim Dungeon spawns behind the player once he's past a certain spot.
All creatures in this spawn are well over CR 20 and some have True Seeing. The creatures and the spawn are fitting for the dungeon, however this specific spawn effectively blocks the exit from a player before he has even had the chance to see what creatures spawn in the area.
Also if you go there with a party, it's highly likely that the some of the party members will be stuck at the middle of the spawn when it's triggered, as it's not that far behind the trigger.
Area: HlimDungeon1