In Blackadder Goes Forth: General Hospital Blackadder reveals that Nurse Mary is the spy. She is lead away to be shot and then George reveals that he was really giving the information away by sending letters to his uncle.
Darling and Blackadder then race to the General to tell him.
Was she not a spy? If not then why didn't she say so and why did she not spot that Hull was not one of the great universities? Also, all of the evidence pointed towards her (speaking German, quizzing Blackadder over tank movements etc).
If she was a spy then why rush to tell the General, she would be shot anyway.