Posh made a nightmare start and were a goal behind after just 30 seconds, Izale McLeod tucking away Mark Marshall's low cross.
And it got worse after 15 minutes when a slip from Gabi Zakuani allowed Ricky Holmes to race through and double the League Two side's lead.
But Frecklington, who was taken off the transfer list earlier this week, netted from former Bees defender Grant Basey's cross before half-time.
And Lee Tomlin spared Posh's blushes early in the second half when he slotted George Boyd's pass into an empty net to secure a draw.
Any reproduction, publication or redistribution of this material without the written agreement of 365 Media Group is strictly forbidden.