Back when I first started playing D&D in ’79, there was a tale that went around about never going into level 7 of one DM’s megadungeon.
It seems that a year before I joined, another group had been ambushed by an acid-spitting eight-headed hydra. They were already in rough shape and the wizard was completely dissolved by the acid on the first round. So they ran. To cover their retreat a blade barrier was cast.
That is, of course, when they discovered it was a lernean hydra (while writing this recollection, I looked through the various Monster Manuals and it turns out in 5e all hydras are Lernean hydras!) and the last group to go to level 7 discovered that the hydra had hundreds of heads now and had mutated and twisted to fill most of the dungeon level.
Fortunately for those still questing in those depths, there was a long set of stairs that lead from level 5 to level 8, and an elevator that connected level 4 to level 9, so the hydralevel could be skipped entirely.
Which isn’t to say we didn’t occasionally find other accessways to level 7… they were easy to identify as one well leading to both level 8 and 9 had hydra heads lashing out from it on both levels.