required variable "url" was not used in form.
I'm curious about this error:
19:54:28: [-] 384/408 required variable "url" was not used in form.
I see it during submissions a lot of times. For example on Drupal and other engines where other people have links, I often get this error. Even on sites I have gotten links before - the error comes and I cannot post again to that site.
What causes this?