Completed a 30m high mixed/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 clipped to a strand on the knot itself on the 'dead' side of the bight near the tail, 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 tail 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