What it says on the tin: When you're editing tags on another image, after having already edited one or more other images before it, there's a "recent" list where you can click on a tag you've added to other images in the same session, and the system automatically adds that tag to the current image's list of tags. ...but they don't have any "-[tag]" entries you can click, so that the tag is automatically found in the list, and removed from it.
If this is possible, it would make for a very nice addition. Would make it so that if you're removing an erroneously-added tag from a lot of images, you could just click it and hit submit. Makes life a lot easier for users who don't know how to run scripts/plug-ins/etc.