If you leave /fly on and warp somewhere, when you return to that world /fly is off and you fall.
Also, I have found that fall damage is extremely heightened when you turn off /fly; you can often die from just falling a few blocks.
I've remedied these issues myself by trying to remember to either always turn /fly off before leaving a world, and being safely on the ground when doing so, or carrying little-no items when travelling between worlds.