If you use a fishing rod on a mob, and fling it high up into the air, and then immediately save and quit, you get teleported to that mob and often fall to your demise. You can save and quit when the mob is really high up, or when it is low to the ground, but both will kill you (unless you have good armor or land in water, etc.) I have no clue why or how it happens, but it made me lose 30 levels. I tested it, and it was not because I was standing on slabs. Solid blocks work the same. You can test it if you want.