In the understandably lacklustre atmosphere, Peterborough emerged the stronger side, with Tommy Rowe sweeping a neat half-volley narrowly wide from a clever knock-down from Liam Dickinson.
The visitors were surprisingly dominant in midfield against the experience of Northern Ireland international Damien Johnson and Argyle's player of the season, Welshman Carl Fletcher, both of whom have captained their respective nations.
Peterborough boss Gary Johnson looked relatively content with the opening exchanges, unlike his previous visit to Home Park in March which resulted in a 3-2 defeat and the end of his reign as Bristol City manager.
Argyle sprung to life with Bondz N'Gala swivelling in the box and hitting a post before Wright-Phillips opened the scoring with a fabulous finish from a difficult angle in the left channel that fizzed into the far bottom corner.
The same man was involved in a superb chance moments later as he danced past full-back Charlie Lee to set up Joe Mason for a low shot that was steered narrowly wide.
Wright-Phillips ended the half with a wonderful piece of control and instant shot that drifted an inch beyond the post.
A painfully quiet second period burst into life when Mark Little fired over a low cross for Mackail-Smith to covert an easy equaliser for the visitors.
Peterborough completed an excellent turnaround six minutes later when Mackail-Smith's neat glancing header drifted beyond Romain Larrieu to give the division's basement club a surprise lead.
Alan Judge nearly capped a fine individual performance with a wonderful free-kick that battered the underside of the crossbar before Kari Arnason slammed the rebound hopelessly wide to complete a horrible campaign for both clubs.