I know - I wasted those minutes trying to find an answer!!!

I filtered by a tag I don't want, then removed the tag from any images it showed up with (the preinstalled images all have whacky tags!).

Next time I looked, that tag was the largest showing - despite a filter showing no images at all with that tag. Now it shows as smaller... perhaps it will waste away from lack of love? Perhaps a reboot would get rid of it?