Peterborough goalkeeper Joe Lewis' late mistake cost his side the chance to go top as bottom club Cheltenham earned a precious point at London Road.
Aaron Mclean broke the deadlock nine minutes before the interval when he converted Craig Mackail-Smith's cross from close range.
Earlier Posh striker McLean had seen a shot rebound off both posts.
Victory looked certain before Lewis allowed a cross-shot from David Hutton to squirm under his body.
However, Cheltenham could still be relegated as early as Monday evening if results, including their own, go against them.
Peterborough: Lewis, Martin, Westwood, Lee, Blackett, Batt (Green 78), Keates, Coutts, Boyd, Mclean, Mackail-Smith. Subs Not Used: McKeown, Torres, Crook, Blanchett.
Goals: Mclean 36.
Cheltenham: Brown, Diallo, Duff, Townsend, Lindegaard (Constantine 78), Finnigan (Bird 46), Gallinagh, Artus (Hutton 62), Berchiche, Connor, Antonio. Subs Not Used: Puddy, Haynes.
Goals: Hutton 66.
Att: 9,817
Ref: Rob Shoebridge (Derbyshire).
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