A stunning equaliser from midfielder Tomoki Iwata dented Hull's Championship play-off hopes as Birmingham fought back to earn a 1-1 draw at the MKM Stadium.
The Tigers looked to be heading for a valuable three points to cement their grip on sixth place thanks to Joe Gelhardt's 14th goal of the season midway through the first half, until the Japan international's sweetly-struck long-range effort flew into the top corner 13 minutes from time.
Defender Semi Ajayi blazed a close-range shot wide to spurn a chance to win it for Hull as stoppage-time approached.
A fourth league game without a victory represents their worst run of the season and leaves Sergej Jakirovic's side two points ahead of Wrexham in seventh, with both clubs having three games left to play.
Hull ended the Blues' long unbeaten home run when they came out on top 3-2 in a stormy encounter at St Andrew's in October and Gelhardt's 24th-minute strike gave them hope of sealing a league double over Chris Davies' mid-table side.
Liam Millar launched the deadlock-breaking move down the Tigers' left. When the Canadian winger was dispossessed by Carlos Vicente as he surged into the Birmingham penalty area, the ball fell kindly for Gelhardt to turn and fire into the bottom corner left-footed with a crisp angled drive from 15 yards, which found the net with the help of a significant deflection off defender Phil Neumann.
After taking the lead through the on-loan Leeds forward's first goal since the 2-1 victory at Wrexham last month, the hosts almost doubled their advantage shortly after the half-hour mark.
England Under-21 international James Beadle came for but got nowhere near Mohamed Belloumi's long throw into the six-yard box from Hull's right. The on-loan Brighton keeper quickly atoned for his misjudgement in the ensuing scramble by clawing a close-range effort from Oli McBurnie to safety.
The Blues were largely anonymous as an attacking force before the break. It was not until half-time approached that they mustered their first real threat on goal, but Ibrahim Osman was unable to keep his header on target after meeting Jay Stansfield's cross from the right a dozen yards from goal.
The second-half followed a similar pattern, with Hull enjoying the majority of the ball and their opponents looking to break on the counter-attack. Birmingham substitute Demarai Gray forced a smart low save from Ivor Pandur with a fierce low shot from distance inside the final 20 minutes.
Hull failed to heed the warning and when they failed to clear another ball into the danger area, Iwata was given time to fire home a memorable third goal of the season from 25 yards.
Ajayi's costly miss soon followed and Beadle's fine save from a stoppage-time McBurnie free-kick snapped Birmingham's four-game losing run on the road.
Hull's Sergej Jakirovic:
"We're aware of dropping those 21 points [from winning positions]. We didn't play our best game today which makes everything very tight. We have to be brave.
"There are three games to go and we'll try to win the points. All three of us (Wrexham and Derby) have two away games so we'll see who will be the most successful.
"We have a lot of problems with team selection in terms of injuries but we have to try everything to take three points in each of those games."
Birmingham's Chris Davies:
"We deserved a point and if anyone could have won it then it was us. I was encouraged by our performance.
"The commitment from the team was there and we showed resilience and character to stay in the game after going behind. We got in some good positions early on without capitalising on those. From our point of view there was a lot more good than bad from the performance.
"To concede their goal was disappointing because there was nothing in it. Our pressing game was good to limit their possession and we deserved our goal when it came. It was a game of few chances for strikers on both sides.
"The pitch was dry and wasn't conducive to slick passing football. Tomoki did ever so well for the goal, he sprinted into position and earned his luck when it dropped for him. He faded it into the top corner with pace and that shot must have been travelling at about 100 miles an hour!"
