Hummmm,

Sewers. This fellow did not mention the noxious / hazardous gasses in confined spaces. Best to let a "Friend" go first. If they pass out, don't go in your self, pull them out with the rope you tied around them. Wait, you did think about getting the body back, right?

Elevator Cables. Ever seen an elevator cable? They are covered in grease. Good luck holding on to that and descending safely.