While Dungeon Nughtmares II is a polished game, more well-made than its predecessor in fact, it is still not free of bugs and glitches. However, there's no need for panic, as they are rare.
Screenshots warning: The screenshots have been tampered with before they were uploaded to the wiki. The brightness (exposure) has been raised to better reveal the objects of interest, for this wiki only. The player is not supposed to see this well when playing the game, as everything is actually much darker.
Map Generation mistakes Edit
There are cases, where the procedural generation algorythm fails and what comes out of this is usually props (tables, chairs) blocking the player's way to a chest, or two chests being too close together and blocking the way. Another particular mistake involves Nights 4 and 5. If a real door is right in the end of a hallway and being straight forward and not on the left or right, a prop can sometimes be placed right in front of it during generation and block it. The player can open the door, but not enter, because the prop is blocking the way. As of 2019, this bug has not been fixed.
Workaround: It is advised to search the rest of the map if you still have time and if you don't find what you are looking for, restart the level.
Skeletons clipping through walls Edit
From the title, this looks like a glitch, but actually, it happens because of a bug, or an exaggeration of a mechanic (probably the latter). It doesn't happen occasionally, but always and mostly because of the player. It is possible to push a skeleton inside a wall and almost the whole skeleton will fit. This is probably an exaggeration of a mechanic that allows the player to reopen the way, if it is blocked by a skeleton that moved to a particular spot.
Clipping through the floor (Worked around mostly by the developer) Edit
It happens very, very rarely, but it is possible for the player to suddenly clip through the floor as they walk or run and fall down to nothingness. If this happens in Night 0 or 1, there's no permadeath, so it won't cost anything more than just causing frustration. If it happens in Nights 2 and 3 though, it counts as falling inside a bottomless pit trap, so there's 50% chance of permadeath. If it happens in Nights 4, 5, 6, 7, no permadeath occurs, because there are no bottomless pits in these levels. Seems like this glitch is there because of that particular version of Unity, so, K Monkey probably programmed falling down in these levels not to lead the player to the doors and instead, restart the level immediately, as a workaround to this glitch.