Sean Dyche admitted his frustration after Burnley were robbed of a point following Arsenal's controversial late winner at Turf Moor.
Dyche's side had gone closest to breaking the deadlock in the second half when Michael Keane's towering header hit the crossbar, but in the third minute of added time Arsenal skipper Koscielny snatched the winner with what appeared to be his elbow.
Koscielny was lurking at the back post and was hit by Alex Oxlade-Chamberlain's close-range effort after Theo Walcott had headed on Alexis Sanchez's ball in following a short corner.
There was also a hint of offside with both Oxlade-Chamberlain and Koscielny standing close to the goalline.
"It's a handball goal," Dyche said. "You're frustrated at any point in the game but you're certainly frustrated when you've not even got a kick left in it, not to get something from the game.
"Apart from that, which is out of our control, which is up to the referee, we delivered a very good performance. Great organisation, I was really pleased tactically, the energy and the belief in the group."
Dyche also felt Koscielny was offside when he turned the ball home.
"There were a number of things," he added. "You can only think the referee's honest enough and will probably have a tough night driving home thinking about things.
"He was good in general and I don't want to harp on about it, but it was a tough one today from the referee in the last six seconds."