With the former England star set to make his grand entrance in next weekend's 'clasico' against bitter rivals Marseille, PSG, chasing a first league crown since 1994, had expected to cruise six points clear of Lyon who showed their own mettle earlier with a 4-0 thrashing of Bordeaux.
Instead, the Parisians slid to their fourth league loss of the campaign to leave the race for the championship wide open.
The capital club, bankrolled for the past two seasons by Qatari cash, now have 51 points from 25 matches for just a three-point lead over Lyon with Marseille just two points further back to add spice to next week's encounter.
Paris turned in one of their all too frequent off-colour showings to underline just what a Jekyll and Hyde outfit Carlo Ancelotti's side are.
Midweek had brought an away win at Valencia to leave PSG with one foot in the quarter-finals of the Champions League but Sunday brought an uncomfortable reality check as Sochaux showed up several flaws in their visitors' armoury.
Beckham was not the only missing piece in the jigsaw as Jeremy Menez, Thiago Silva and Thiago Motta were all out of action, but they did not lack flair and potential match-winners in the shape of Zlatan Ibrahimovic and Javier Pastore.
Alex Costa gave PSG the lead on the half hour with a header from a Pastore corner.
But Sochaux roared back and Sebastian Roudet smacked home the equaliser left-footed four minutes later.
Ten minutes after the restart Giovanni Sio, their new French-Ivorian signing from Wolfsburg, scored a beauty after a one-two with Roudet.
Having already slain Marseille at home earlier this year, Sochaux were good value for their lead but on 76 minutes PSG levelled through Mamadou Sakho, who touched on another Pastore corner though home defender Jerome Roussillon appeared to get the final touch over the line.
But with six minutes remaining, Cedric Bakambu fed Algerian midfielder Ryad Boudebouz down the right for Sloan Privat.
The French Guiana-born striker nodded back for Bakambu, who forced home the winner.
Elsewhere Sunday, Saint Etienne saw a four-match winning run come to an end with a 1-1 draw at Reims which allowed Nice, 1-0 winners at Bastia on Saturday, to move above Les Verts in to fourth spot.
Lyon earlier ended a run of three successive defeats by thumping Bordeaux with youngster Clement Grenier scoring a brace and fellow up and coming stars Gueida Fofana and Alexandre Lacazette adding the others in an impressive away stroll.
Victory saw Lyon regain second spot from Marseille, who had gained a barely-deserved 1-0 win at home to Valenciennes on Saturday, with an injury-time goal from defender Rod Fanni.