Northampton goalkeeper Shane Higgs played his first 90 minutes since the start of the year in a practice match at Sixfields.
Higgs has not made a first-team appearance since January 2 after rupturing an Achilles tendon in training.
However, the 35-year-old's presence could not prevent a side whose only other first-team experience came in the shape of Lewis Wilson, Anthony Charles, Louis Moult and Henoc Mukendi losing 2-1.
Peterborough, who included the likes of Joe Day, Scott Griffiths, Lee Frecklington, Emile Sinclair and Daniel Kearns in their ranks, went into half-time in control following Jonson Clarke-Harris' 22nd-minute strike and and a Frecklington penalty.
Moult reduced Northampton's arrears five minutes into the second half but Posh held on for the victory.