Stairs down can lead to unsafe terrain
There are complaints that going down a stair can lead to magma, chasm and so forth. And at worst can result in death. This is a dungeon generation bug.
However, there is talk of keeping this as a feature and at least in the interim, it makes sense that the player can see the stairs leads to unsafe terrain before descending (it's not like the stairs are an illusion and they're forced to step out onto the chasm). To this end, this issue should add a confirmation dialog to the process of descending to unsafe terrain (chasm, magma..).