Derby County looked to have secured their third league win on the trot when they had taken the lead in the last minute of regulation time.
But the whole ground was left standing in amazement after Poom, a Rams player for six years, sprinted the length of the field to head home with virtually the last kick of the game.
A point was the least Sunderland deserved in truth from a game in which they enjoyed the greater share of chances.
They opened with an aerial barrage which produced opportunities for both Kevin Kyle and Marcus Stewart.
And the constant running of John Oster consistently troubled the Rams rearguard as the home side clung on to equality.
But as the second half progressed Derby's youngsters, with the two 16-year-old's Tom Huddlestone and Lee Holmes showing maturity beyond their years, began to establish a foothold in the game.
Chances were spurned at either end before the match moved into its breathless finish.
For the second game running captain Ian Taylor scored for Derby with a splendid volley. His six-yard strike with the outside of his boot may not have quite matched his brilliant goal against Watford but the roar which greeted it spoke volumes for its worthiness as a winner.
But little did the home fans realise that their former favourite would become such an unlikely saviour when he leapt to head home Sean Thornton's 94th minute corner.
Both managers could only admire the drama. George Burley said: "For me, it's the goal of the season. There is a saying that old players come back to haunt their clubs but you don't expect it from the goalkeeper."
And Sunderland boss Mick McCarthy labelled his keeper, "awesome", and added: "I have never seen anything like it in all my years in football. It was an unbelievable header and the lad truly deserved it."
Derby: Oakes, Jackson, Mills, Caldwell, Boertien, Morris, Taylor, Huddlestone, Holmes, Junior, Svensson. Subs: Grant, Zavagno, Elliott, Valakari, Labarthe.
Sunderland: Poom, Williams, Bjorklund, McCartney, Arca, Oster, Thirlwell, Healy, Butler, Kyle, Stewart. Subs: Ingham, Clark, Proctor, Thornton, James.
Referee: P Walton (Northamptonshire).