When is a shoe like a face? The answer (or one answer, at least) is in the study:
“Statistical Discrimination of Footwear: A Method for the Comparison of Accidentals on Shoe Outsoles Inspired by Facial Recognition Techniques,” 
Nicholas Dominick Koslap Petraco, Carol Gambino, Thomas A. Kubic, Dayhana Olivio and Nicholas Petraco, J Forensic Sci, Vol. 55, No. 1, January 2010. The authors report:
“it is a widely held belief that patterns of accidental marks found on footwear and footwear impressions possess a high degree of ‘uniqueness.’ This belief, however, has not been thoroughly studied in a numerical way using controlled experiments. As a result, this form of valuable physical evidence has been the subject of admissibility challenges. In this study, we apply statistical techniques used in facial pattern recognition, to a minimal set of information gleaned from accidental patterns.”