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

Tag changes for image #13538

Image		Tag	Action	Timestamp	User	Retained?
13538		+-SH hornless unicorn Unwatch Watch Unspoiler Spoiler Unhide Hide Sign in to Watch Filter 48	Added	4 days ago	Heat Sink Stop! This user is a staff member. Ask them before reverting their changes.	Yes