Completed a 30m high sport climb in the middle of nowhere in Australian bush that had a two hour walk-in and poor route finding due to vegetation. Upon tying off on the lower off and doing a figure-8 on the bight and clipping a carabiner and then to my harness, I lowered down as is normal on sport routes off on the in situ gear. When I got to the bottom I realised that the carabiner attached to my harness was not attached to the loop of the figure-8, it was in fact attached to one of the top threads that was only being held in the knot due to the friction of the rope. If it was a new slick rope and the knot wasn't pulled as tight, the thread would have easily slipped through when I weighted it and I would have continued the 30m drop to the floor. I was hot, dehydrated and probably complacent because I've done it hundreds of times