Peterborough's January arrival George Boyd climbed off the bench to snatch a late draw and deny Boston a valuable win in their battle against the drop.
The injury-ravaged Pilgrims looked on course for an unlikely win when Nathan Joynes seized on a defensive mix-up to fire them ahead in the 61st minute.
However, £260,000 January signing Boyd levelled when he beat keeper Andy Marriott with a superb 35-yard volley.
Craig Morgan then saw a stoppage-time strike hit the bar as Boston held on.
Peterborough: Tyler, Newton, Morgan, Arber, Blackett, Low, Hyde, Strachan (Boyd 67), Day (Gain 66), Crow, Mackail-Smith.
Subs Not Used: Butcher, Futcher, Jalal.
Goals: Boyd 75.
Boston Utd: Marriott, Thomas, Ellender (Talbot 32), Stevens (Maylett 86), Cryan, Cooksey, Vaughan, Farrell, Jarrett, Broughton, Joynes, Talbot (Clarke 78).
Subs Not Used: Rowntree.
Booked: Vaughan, Farrell, Thomas, Joynes, Broughton.
Goals: Joynes 61.
Ref: A Hall (W Midlands).