I am sorry if this should be a duplicate, I only found MC-55686, which was maybe caused by this.
Relates to: MC-55686
When you clone move a sign with text (maybe this affects also other NBT storaging blocks) and then place a sign at the old spot, the sign will contain the text of the old sign.
Affected blocks:
Sign (Text)
Command block (Last Output)
Command used:
/clone ~ ~ ~ ~1 ~-4 ~100 ~ ~-4 ~2 replace move
You have to place on the side of the wall facing in the x-Direction a sign with text and then on top of the wall a command block with this command
It seems like this only happens to the latest cloned signs, as soon as you clone move it again, the old ones won't work again
Linked issues
is duplicated by
Attachments
Comments

If I would have used a sign with NBT stored, I wouldn't be able to change the text 😉

Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w43a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.
Did you ever use ctrl + pick block on the sign?
If so, that would explain why the new sign had the same text as the old one.
Could not replicate in 1.8-pre4.