This is the forum for helping you out if you're stuck in Grimrock 2 or if you want to discuss about the mysteries of the Isle of Nex. Warning: forum contains spoilers!
In the skyway room, I remove the items from the pressure plates so that the door opens and I can go up the ladder to look down on that same room. I drop a rock and a sparkle floor tile appears in front of me. As soon as I move forward I fall through the tile and back to the room below. Is this a bug? Or am I doing something idiotic?
Yeah this is a bug. it happens when you have stones equipped. Just equip something else and leave the stones in your inventory so that you can solve this riddle.