Middlesbrough boss Tony Mowbray described the 3-2 defeat by Birmingham as a "big blow".
Boro missed the chance to go level on points with Cardiff at the top of the Championship after twice throwing away a lead and Mowbray could not hide his disappointment.
He said: "We came to win tonight and tried to be positive. We got our noses in front twice and it's tough to take when you're in front feeling reasonably comfortable.
"I think the nature of the goals were obviously disappointing for us. I don't think they had to work too hard for any of their goals and that's the most disappointing thing for us.
"I haven't seen the penalty again but I've told (Andre Bikey) you can't hang a leg out in the box, you have to defend properly, stay on your feet and push people from goal.
"I think the timing of the penalty gave them a little break because we were in the ascendancy and disappointed not to be two goals up coming into half-time, so to come in at 1-1 was a big blow for us."
Both teams had their chances, but ASSE will be happier with a result that extends their unbeaten run to 10 matches and moves them two points behind leaders Lyon.
Ajaccio, meanwhile, have won just once in their last eight fixtures and sit two points above the relegation zone.
Despite their run of poor form, it was the hosts who made the better start and forced the first save after 12 minutes with Stephane Ruffier turning away an angled drive from Chahir Belghazouani.
St Etienne were made to wait 10 more minutes for their first opening. Max-Alain Gradel sparked the move with a cross from the left which produced a testing volley from Romain Hamouma and a fine save from Ajaccio goalkeeper Francisco Ochoa.
Ruffier then returned to the action five minutes later, diving low to keep out a 25-yard shot from Mehdi Sbaa as the home side threatened once more.
And Adrian Mutu had another good chance for Ajaccio five minutes before the interval, with the Romanian striker getting on the end of a neat pass from Samuel Bouhours before Ruffier again intervened.
Gradel tried to give St Etienne the advantage early in the second half but directed his 30-yard free-kick narrowly over the crossbar.
Another opening followed for Ajaccio after 65 minutes. Sigamary Diarra supplied a cross into the area which was headed over the target by Belghazouani.
Ochoa did well to get down low and block a left-footed shot from Renaud Cohade moments later as St Etienne pressed for the opening goal.
But it was Ajaccio who came closest to snatching the points in stoppage time when Belghazouani saw his snap-shot from 20 yards cleared off the line by Loic Perrin.
