Michael Bostwick's equaliser earned a point for Peterborough as they missed the chance to escape the Championship's bottom three.
Danny Swanson's first goal of the season gave Darren Ferguson's side hopes of a win over Charlton.
But the visitors struck back with two goals in four second-half minutes from Johnnie Jackson and Danny Haynes.
Bostwick's spectacular 25-yard effort ensured a draw, but Posh are two points from a position of safety.
Peterborough, chasing their fourth win in six league games, had an early escape when Lawrie Wilson hit the bar from 15 yards, before Ricardo Fuller also went close.
Chris Powell's mid-table Charlton side had taken early command with Fuller going close and Haynes' long-range effort being caught by Peterborough keeper Robert Olejnik.
Posh then perked up as Gabriel Zakuani fired over from six yards, before Swanson scored from the edge of the penalty area.
They could have made it two before half-time when Nathaniel Mendez-Laing's shot was saved by David Button.
And they were unlucky a minute into the second half when Lee Tomlin struck the post.
Charlton then levelled when Callum Harriott supplied a pass to Jackson who finished from 12 yards out.
Four minutes later, Haynes got on the end of a Michael Robinson header to nod in from six yards.
Button then denied Gayle, but could do nothing as Bostwick thumped in the equaliser.
Peterborough manager Darren Ferguson: "In our situation any match we fail to win goes down as a missed opportunity - that's certainly the case tonight.
"I viewed this week, with Charlton at home tonight and Ipswich coming here on Saturday, as a great chance to put six points on the board.
"The most we can get now is four and that makes Saturday the biggest game of our season. It is a must-win if we are serious about staying up."
Charlton manager Chris Powell: "We were well aware of the fact that Michael Bostwick has a good strike on him. It's something we spoke about beforehand but unfortunately we couldn't stop him unleashing one to get Peterborough level.
"Both teams had chances and both could have won it, but I'll settle for a point after a couple of poor results lately.
"We're only six points clear of the relegation zone so we can't afford to be complacent."