Trailing 3-0 after 60 minutes courtesy of Brett Pitman's stunning hat-trick, Wanderers were thrown a lifeline when Andy Savage converted a 71st minute penalty.
Giambrone reduced the deficit further in the 82nd minute before Ikechi Anya equalised three minutes later to send the game into extra-time.
Both sides had chances to win the tie, but Wanderers completed the most unlikely of comebacks in the 103rd minute, diverting the ball past hapless Cherries keeper James Rae.
One to watch: Brett Pitman (Bournemouth)
The Jersey born striker showed he is the cream of the crop on the south coast with another polished display.
After planting five goals past Cirencester at the weekend, the 17 year old provided further evidence of his talent by scoring a treble against Wycombe. Pitman also netted for the Cherries' senior side in the LDV Vans Trophy earlier in the month. Defenders take note.
From The FA's Web Site