When I rename a minecart command block, place it and leave my game in single player, the minecart name becomes @ when I come back. I'm not sure if the same thing happens on servers!

Steps to Reproduce:
1. Rename a minecart command block in an anvil
2. Place it and leave your world
3. Come back to your world and it will now be named @
(You can test that by using command /say @e)