In "Maggie Had a Little Pigpen" in the May 1991 Word Ways, Peter Newby pointed out that if a 3-by-3 word square was encoded using the pigpen cipher and inverted, it might be possible to read off another 3-by-3 square upon decoding.
Word Ways: Vol. 25
, Article 19.
Available at: https://digitalcommons.butler.edu/wordways/vol25/iss3/19