Jabo Ibehre notched the winner for Walsall which dashed MK Dons' hopes of automatic promotion.
Ibehre turned Jude Stirling inside out before firing in from 25 yards to hand automatic promotion to Peterborough and condemn MK Dons to the play-offs.
Walsall keeper Clayton Ince twice denied Sam Baldock in the first half before Ibehre struck on the hour mark.
Ince proved a barrier in the second as he saved Mark Wright's header and then kept out Baldock's low free-kick.
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