Description
When you reach a level above y=256, physics are inverted; gravity goes upwards, so you float to a certain point. That point is actually defined by the inverse of the ground level.
I took the example of having the highest ground level of 70. If you teleport from the same position to the block limit, 256, using /tp ~ 256 ~, you will not fall. Instead, you will "fall" upwards to level 326. The reason for that limit is because 326-256=70. So, you stop at that position because you are supposedly touching the ground.
Here's a video of that example: http://momoperes.ca/videos/velocity_bug.mp4
This does not seem to affect dropped items, but they seem very glitchy too. If you drop one under you while floating, you will be able to pick it up while it is 30 blocks under you.