If you rename a sign using an anvil and place the sign anywhere, when you break the sign, it is named "Sign" rather than whatever you previously named it to.
MC-49119
Named signs losing names when placed
Resolved
Duplicate
0
Gary Woodward
(Unassigned)
Unconfirmed
Minecraft 14w07a
Retrieved
This is the same for any "Block" that can be placed after naming in an anvil.