Bug: when you move a sign by cloning it with the "move" attribute, the sign is moved. However, when you attempt to place a new sign to where the sign once was, the sign automatically gains the NBT data of the other sign.
How to recreate: Place down a sign and write some stuff on it. Then move it using the /clone command and adding "replace move" to the end. Then attempt to place down a new sign to where the previous sign once was. It will not display an empty text field like a regular sign, but rather display a text field filled with the NBT data of the old sign.