Description
They can contain underscores, and they can sometimes come in pairs, and that is the wiki syntax for underline. If the token happens to be in a block that is supposed to allow wiki syntax, which happens almost always, then the whole HTML gets broken, as in:
<input name="form_token" type="hidden" value="M5"></input><del>40xQ_xNlw9am5bA9RA" /></del>