Lewis contrived to let a last-gasp drive from Cherries full-back Adam Smith creep under his body as Bournemouth completed a stirring second-half fightback in the dying moments at London Road. Earlier a couple of goals from League One top scorer Craig Mackail-Smith had left them on course for a victory that would have lifted them to within a point of the automatic promotion places.
Mackail-Smith broke the deadlock in the 11th minute when collecting a searching Grant McCann pass and firing past Shwan Jalal from the edge of the box, admittedly with the help of a deflection, but Posh didn't need any assistance to double their advantage four minutes later as Tommy Rowe thumped past Jalal after bursting into the box.
Bournemouth halved their arrears 10 minutes before the break when Danny Ings poked the ball home after Lewis had flapped at a corner, but the two-goal cushion was restored three minutes before the break when Mackail-Smith pounced from point-blank range after Ryan Bennett had flicked a McCann corner into his path.
It stayed that way until veteran substitute Steve Fletcher breathed life into Bournemouth with a fine header from the tightest of angles after being picked out by Liam Feeney's deep cross with 19 minutes to go.
That strike set up a nervy finale which came complete with a late twist as Spurs-loanee Smith grabbed his first Cherries goal. There was still time for Posh to win it at the other end but McCann slammed a volley down into the ground and over the bar.