You guessed it, it's another Rules update. Check here for more info

Tag changes for image #21718

Image		Tag	Action	Timestamp	User	Retained?
21718		+-SH horn Unwatch Watch Unspoiler Spoiler Unhide Hide Sign in to Watch Filter 12514	Added	2 days ago	Heat Sink Stop! This user is a staff member. Ask them before reverting their changes.	Yes