When you take a mine cart ride and fall onto a mine cart track while flying over a solid block before said track (yet still landing on the track without touching the solid block ever) you will take fall damage later in the ride, often after you fall a distance again to another track (without touching solid blocks) or when you near the end of the current track. Evidence can be found in Hat Films' Epic Hatventure:
http://www.youtube.com/watch?v=1_xNbRppM48
It's been around since before 1.1. Similar bugs:
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Don't link to a 20-minute video clip without telling us when in the clip the bug occurs.

It happens from 10:50 to 13:40. Note that they die before their minecarts ever land. I can also make you a small map that has it without lava as to not confuse it with the other bug.

Problem being that this bug doesn't have any relation to lag client or server side in terms of network activity. Here's a video with JUST the issue:
http://www.youtube.com/watch?v=YiO9KzCSvbs
Make sure to watch the whole thing, as the first fall will not do damage. Subsequent falls WILL however BEFORE you actually fall with the Minecart. (Added a world download to the video.)

X = -97.4
Y = 69
Z = 172.1
for the start of the minecart track to test.
Aha, thanks for the video clip and test world. It's fixed now

This is caused by server-side or client-side lag. Should allocate more ram to fix.

The x is broken.(If 'blocks' is1 1 1, x,y,z is 0 1 1)
and When I use 'kill' my skin become steve.
Also, when I broke the tall(2blocks)plants's bottom, it's skin change to sunflower and return to original plant's skin in short time.
Sorry. I can't speak English well.I want you guys change this bug.I hope you can understand this writing. Please, check this bugs.
this bugs are in 14w21b