I put a minecart with command block with no command on a activator rail and placed a redstone torch next to the activator rail, which promptly kicked everyone on my server and prevented people from joining it. The message was "Internal exception: java.io.IOException: Received string length lower than maximum allowed (88 > 64)
Duplicate of MC-33182 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.