When you rename a cart with a chest, furnace, tnt, or hopper and you break them your item (not the cart) won't be renaimed.
Related issues
Comments
migrated
MC-12132
Renaming Carts
Resolved
Duplicate
0
migrated
migrated
(Unassigned)
Unconfirmed
Minecraft 1.5
Retrieved
Duplicate of MC-1981, please use the search function to see if your bug has already been submitted. Currently over 54% of tickets are being closed as duplicate.