You've just met burning ropes. You can usually tell if it's burning when you're partway down, so just jump before you get down to that portion of it. No, those won't harm you when you walk back past them again. Just don't jump until you're well past them. That way you won't accidentally grab the burning end. Granted this does mean you can't climb back up if you miss something on the level above, but if you manage your items well you shouldn't have to worry about that. As I said though, you won't be hurt by the rope when you're walking by it unless of course you happen to make a bad jump and accidentally grab hold of it.