When the Minecraft client pings a server from the server list it doesn't close the connection properly when it has received the status response from the client. When it receives the status response, it sends the packet to calculate the ping to the server, but if the server sends the status response again after that it won't close the connection - instead it will just infinitely resend the ping packets until the server list is closed again.

Basically the Minecraft client shouldn't allow sending the status response more than once from the server, and should close the connection if this happens. While it allows nice effects from the side of the server (e.g. animated favicons or animated MotDs), from the side of the client keeping the connections open infinitely is bad, especially if you have a lot of servers in your server list.

This could be fixed easily by storing the current state somewhere, and validating it when receiving the packets.