When using the 1.7.2 client, the Multiplayer server list shows servers as always "Pinging..." and won't finish pinging. If you attempt to connect to one of these servers, you will forever be "Logging in..." and the server that you're connecting to sometimes gets a message stating "Disconnecting /XXX.XXX.XXX.XXX:52136: Protocol error" (I censored my IP though). It usually just says "/XXX.XXX.XXX.XXX:52136 lost connection" though. If the server is down, it will say "Can't connect to server." instead of "Pinging..."

This is somewhat similar to https://mojang.atlassian.net/browse/MC-36761, except in that it can see the version of the 1.7.1 server.