Mclean had already struck twice against his first professional club but fluffed his lines at the death when driving his second spot-kick of the night against the legs of Orient keeper Jason Brown.
Mclean had earlier broken the deadlock on a dramatic night when converting a controversial 18th-minute penalty after Brown had harshly been adjudged to have brought down George Boyd.
Mclean soon missed a glorious headed chance to double the lead and that proved costly when Alex Revell glanced in his 11th goal of the season from Dean Cox's cross two minutes before the break.
Mclean restored the Posh advantage in the 61st minute by grabbing his 13th goal of the season after Brown had slipped when attempting to reach an Arron Davies delivery, but Posh were again pegged back with four minutes remaining.
Orient midfielder Jimmy Smith chose the perfect time to grab his first goal of the season when racing onto a Cox pass and slotting past home keeper Joe Lewis before Mclean was denied in the closing stages by Brown's spot-kick save.