When an Item is renamed from an anvil it will return with the characters being #.  It will display the new name in the anvil when changing the text but alters the name to # when you remove it from the anvil. 

For example, putting in any item and trying to rename it, "Test" will return #### as the new item name after you remove it from the anvil. 

I tried with multiple different items including name tags and even tried reloading the world.