Jesse Mogan

@Ethan Brown, if you have access to the server you're using (and I'm assuming you do since you're using a snapshot), you can just revert to the previous snapshot (13w38c I believe), and your command blocks will work again, at least it did for me.

I'm sure they'll work out the error in the new snapshot in short order.

Mine has nothing to do with minecarts, it's specifically testing for any player within a three block radius of the command block and I get the same output error.
My command is a simple: testfor @p[r=3], the error I get is [timestamp] /testfor is only usable by commandblocks with analog output.
This same command was working fine in the previous snapshot.