When entering a boat you will get kicked (from the server) with a nullpointer exception and can't join again.
Entering Minecart on Server Causes Perpetual Internal Exception
error when getting in minecart on mutiplayer
Enter a boat or minecart you crash on multiplayer.
unable to join 13w16a multiplayer servers.
Entering minecart causes null pointer
Consistent getting kicked from server when attempting to ride minecart/boat and presumably pig saddle
Crash when you get in Minecart
Mounting boat kicks player
13w16a: Getting in a Boat Crashes Game
Unable to get onto server
Attempting to ride horse crashes game in SMP
Kicked upon entering a server when riding an object
Crash after entering boat on server
Players including myself cannot "ride" entities.
Crash with "NullPointerException" when going into a minecart.
Null Pointer Exception When Attempting to Ride a Minecart
Minecart trashes player
NullPointerException when mounting minecart
java.lang.NullPointerException if jump into minecart on SMP
player mount kicks you from multiplayer server
SMP/LAN: NPE when right clicking horses