Rangers boss Mark Warburton was furious with the award of a free-kick from which Aberdeen scored their winning goal in a 2-1 victory at Pittodrie.
After James Tavernier was penalised for a last-minute challenge on Jonny Hayes, James Maddison curled home a fantastic set-piece from 25 yards.
"The referee handled a difficult game very well," said Warburton.
"He did a really good job up to that point, but to give that one there is frustrating beyond belief."
Warburton is convinced Tavernier made an "excellent tackle" and confronted referee John Beaton on the pitch at full-time.
"I will be careful what I say about the free-kick," he added. "Everyone saw the ball move. The fourth official on the sidelines saw the ball move and made a comment. And yet a free-kick is given.
"You have got to be, I think, 100% sure in these type of games, in those areas of the park.
"Late in the game, when your players are fatigued, I thought Tav made an excellent tackle there. I don't think their player moaned about it and it is a huge decision to give."
'We are getting better'
Rangers dominated the first half as the teams met for the first time since January 2012.
However, the visitors were caught cold in the opening minute of the second period when Hayes broke clear to score.
A surging run from Rangers captain Lee Wallace was halted by Hayes, leading to a penalty converted by Andy Halliday.
Maddison's stunning strike left Rangers in the bottom half of the table, with just two wins from their seven league outings, but Warburton insists his team are moving in the right direction.
He said: "We dominated the ball first half completely but we went in at half-time frustrated it finished 0-0.
"We gave away a schoolboy goal and for three or four minutes we looked a bit rocky. We lost our composure then got the ball down, then played. We got our rewards via the penalty and I thought there was only one team in it.
"I am never happy to drop a point, never mind three. But we moved to the next level as a team and we are getting better.
"The boys are settling in and we were delighted with long periods of that game where we completely dominated."