I was on a server in the Nether. I placed a minecart on the tracks and as soon as I right-clicked the minecart, I was booted from the server with this error:
Connection Lost
Internal exception: java.lang.NullPointerException

And now every time I try to log into that server, I get that error immediately. I know the server still works because others are still logged in.

I believe it is because my player is in the minecart, and since Minecarts/boats are now persistent with the character (You remain in the minecart when you log back in), I am being booted immediately because I'm still in the minecart...