Summoning a Boat/Minecart/Horse/Pig and giving it any entitydata while riding it will cause it to teleport to the void under x=0 z=0. Riding the entity while looking at its entitydata states there is no entitydata saying "The data tag did not cahnge: {}." What I expected was for the rideable entity to remain where it was but effected by entitydata normally.

Test:
1. Summon Boat, MinecartRideable, Horse (Saddled and Tame), Pig (Saddled)

2. Ride one at a time while doing (3.)

3. /entitydata @e[type=type of entity]

4. Be teleported to x=0 z=0 and in the void. (Optional: /tp ~ 60 ~ to survive and look at coords)

5. Try testing for one of the entities you have sent to the void and it might say the entity is not found, even though it is there. (Normally it won't say anything)