Using the down stairs in Crypts -3 consistently generates an array index out of range exception without changing levels.
It also produces another few bugs. Even though the next level doesn't load, the dungeon level is decremented every time you try to go down (so Crypts -4, -5, etc.). If you try to use the up stairs and the game thinks you're at Crypts -5 or lower, it will do the same thing in reverse..
The level becoming wrong doesn't matter much by itself since you can just go up the up stairs repeatedly to correct it, but the third bug is bad. If you save and quit the game when the game thinks you're at Crypts -4 or less, trying to load the save will generate the same index exception, and the game will be never stop loading. Maybe some other bug in the future will save you on a level that doesn't exist, so the loading process should be more cautious about bad savegames.
At Crypts -4, using the up stairs will teleport you to the down stairs, since you never actually left the Crypts -3 map.