There's an activity I as a tagger sometimes think of as 'tag defense'. Tag defending is when I come up on an overused tag and start stripping it off things that shouldn't have it so as to keep the definitions tight. Some tags tend to creep if not trimmed.
As an example, the hooves tag tends to spread throughout the my little pony pictures. Although the characters are stated canonically to have hooves, the default artistic style of MLP doesn't draw them in, and tagging a show-style picture with hooves is wrong. The tag has to be kept trimmed back so it stays useful. An image such as post #270565 clearly shows hooves. An image such as post #121151 clearly doesn't. (Both images are safe and mostly harmless.)
Other examples include the way the anthro tag keeps trying to creep back into being applied to everything on the site, and the way some pictures end up tagged straight, gay, or lesbian because the artist/uploader/fans know the orientation of a particular character, when that orientation doesn't apply to the picture at all.
What are some other tags that often need defended?
Updated by Waffuru