A controversial penalty scored by Aaron McLean earned Peterborough a scarcely-deserved victory over Grimsby.
Ciaran Toner put the visitors ahead from the penalty spot on 59 minutes after defender Kieran Charnock handled.
But substitute George Boyd hauled Posh level five minutes later with a swirling effort from the touchline that sailed over keeper Phil Barnes.
McLean snatched victory when Mike Dean ruled that Peter Till fouled Jamie Day, who seemed to be stumbling already.
Peterborough: Jalal, Morgan, Westwood, Charnock, Newton (Boyd 46), Lee, Hyde, Keates (Whelpdale 61), Day, Mclean, Mackail-Smith (Howe 84).
Subs Not Used: Tyler, Strachan.
Booked: Lee, Charnock.
Goals: Boyd 65, Mclean 80 pen.
Grimsby: Barnes, Bennett, Fenton, Whittle, Newey, Till (Jones 83), Bolland, Boshell, Toner, Clarke (Taylor 75), Rankin (North 56).
Subs Not Used: Montgomery, Bore.
Booked: Toner, Newey, Till, Boshell.
Goals: Toner 59 pen.
Ref: Mike Dean (Wirral).