When you enter a ridable object, minecarts, boats, horses etc. You get kicked with the message:
Internal exception: java.lang.NullPointerException

This will happen every time you log into that server, untill your player.dat is deleted from the server folder

I took a screencap of the launcher where the bug was

I know there have been made a few of these, but i just wanted to make it clear that it was all ridable objects