With 15 places separating the sides before the clash at the Stade Geoffroy-Guichard, things initially seemed to be going to form - Landry N'Guemo giving the hosts an early lead with a stunning strike from the edge of the area 18 minutes in to punish some slack defending from Antoine Kombouare's side.
It was the Cameroon international's first Ligue 1 goal since December 2013 and, when Yohan Mollo added a second before the hour mark, the result seemed wrapped up.
After the visitors failed to clear the ball, Fabien Lemoine found Erding who squared to Mollo for a simple tap-in.
However, two goals in 10 minutes after the hour turned the game on its head as Yoann Touzghar pulled one back with a long-range drive past Stephane Ruffier.
Tunisian forward Touzghar has been at the centre of an apparent disagreement with Kombouare over his omission from the national squad for the Africa Cup of Nations but, when Kevin Theophile-Catherine handled in the area, Touzghar converted to draw his side level.
With Lens still battling French football authorities over financial issues surrounding last season's promotion to Ligue 2, substitute Pablo Chavarria appeared to have sealed a morale-boosting win 13 minutes from time.
However, Erding spared Saint-Etienne's blushes with a well-taken late equaliser, although Christophe Galtier's men have now won just once in their last six outings and are drifting in the race for the Ligue 1 title.
