This linked video should make things clear enough:
http://www.youtube.com/watch?v=o5LdYDlUink&feature=youtu.be
As you can see, when you watch a player ride a boat in multiplayer, the player sometimes seems to "jump" to a position outside the boat, leaving an empty boat. This happens REGARDLESS of the distance between the players, and wether the boat is moving or not. I have tried this on different worlds, still happens.
Note that the player himself is able to ride the boat normally, and the boat itself acts as usual. It is just the player riding it that, when viewed by other players, seems to jump and disappear.
It is easy to reproduce: Just start up a multiplayer world, and watch someone ride a boat!
UPDATE: The same bug occurs when riding a minecart.
Linked issues
duplicates 1
Comments 2
Sure, the same thing happens with minecarts, but i would not view this as a duplicate of MC-1291. That report was about people turning invisible AFTER leaving the minecart. What I describe here is people lagging and disappear WHILE riding it. Clearly not the same thing, and therefore this should not be put down.
On another note, I did some more testing and it would seem that the place to where the player seems to "jump" is the place where he got in the boat/minecart. Might make this easier to solve.
Duplicate of MC-1291.