Birmingham looked to be heading to their second away victory in six but substitute Hammill popped up with almost the last kick of the game to level in what is scheduled to be the last match of his loan spell before returning to Wolves.
Birmingham's Academy graduate Callum Reilly looked to have settled matters from 10 yards with his first goal for the club on the stroke of half-time.
Despite the late point, Grayson has much to worry about after a lacklustre display which rescued a last-gasp point but keeps Town winless in 11.
Grayson handed a debut to new loan signing, and former Birmingham midfielder, Neil Danns - the only switch from last weekend's rare FA Cup victory at Charlton.
The first opening arrived midway through the half but Butland was equal to Sean Scannell's acrobatic kick from Danns' run and cross.
Birmingham came into the game and were rewarded for their pressure in first-half injury time as Burke's cross was headed back into the path of Reilly who smashed home from 10 yards.
Huddersfield were twice thwarted by Butland at the second half wore on. Substitute Alan Lee was first to be denied as his close-range effort was parried away, with Beckford seeing his header well kept out from the subsequent corner.
But Hammill rescued a point for the hosts in the 93rd minute when he beat Butland from 12 yards.