Shane Blackett's disastrous own goal handed in-form Southend a fifth win in six games.
The Posh defender sliced a low Franck Moussa cross into his own net in the 63rd minute.
Peterborough had taken the lead on six minutes when Dean Keates scored from the spot after a foul on George Boyd.
But Southend levelled against the run of play thanks to a glorious overhead kick from Jean-Francois Christophe before Blackett gifted them victory.
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Booked: Keates, Boyd.
Goals: Keates 6 pen.
Booked: Christophe.
Goals: Christophe 25, Blackett 63 og.
Att: 7,341
Ref: Andy Haines (Tyne & Wear).
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