There's also the bizarre [oc:only](https://derpibooru.org/tags/oc-colon-only) which mostly keeps popping up because people misspell [oc only](https://derpibooru.org/search?q=oc+only). But the mods can't exactly get rid of "oc:only" because there actually is an OC named Only: >>2053197 (and it's just as high-quality as you'd expect).