Boss Tommy Wright claimed referee Don Robertson cost Kilmarnock the chance of Scottish Cup glory in their penalty shootout defeat by St Mirren at Rugby Park.
In the 119th minute, with the home side leading 3-2, Robertson judged Killie keeper Colin Doyle to have fouled Jake Doyle-Hayes inside the box and Jamie McGrath levelled to take it into penalties, with the Buddies emerging 5-4 victors after Jak Alnwick saved Rory McKenzie's effort.
Wright said: "Let's be honest, Don Robertson put it into penalties with a poor decision. When he is man enough he will own up to it.
"I will go and see him after the game. Over the years I have been in to see Don more than any other ref.
"Don Robertson changed the whole game.
"We could have dealt with the situation better, (Ross) Millen could possibly just put his foot through it, but it is not a penalty all day long.
"The goalkeeper clearly gets the ball, the fact that it spills out is irrelevant. No penalty. But Don wanted to have the final say and he got it.
"It is extremely disappointing because there is no doubt we were the better team.
"Their keeper had more saves to make, we were the dominant team and we go out because of a poor decision and that can happen in cup football."
It had been quite a game.
Doyle allowed a harmless cross from Ilkay Durmus to go through his hands in the 25th minute but a header from loan defender Zeno Ibsen Rossi in the 36th minute and a Greg Kiltie strike just before the break gave Killie the interval lead.
With six minutes remaining Buddies skipper Joe Shaughnessy headed in a dramatic equaliser to set up extra time, which Killie played a big chunk of with 10 men after substitute Nicke Kabamba with the Ayrshire side having used all four substitutes.
However, Millen scored from the spot in the 101st minute before McGrath's contentious leveller.
It is St Mirren's second semi-final of the season after losing to Livingston on the semi-finals of the Betfred Cup.
Manager Jim Goodwin said: "It's a huge relief.
"I felt it very hard to celebrate at the end the way I probably would in a normal situation.
"We made extremely hard work of it and we picked a terrible night, so poor defensively. The goals we conceded are just not like us, it's as simple as that.
"It's something we're going to have to rectify before the next league game and then before the trip to Hampden to play St Johnstone.
"Two semi-finals in one season, it's a great achievement for everybody.
"I'd much rather do it easier than we did tonight.
"But no one will remember the performance in a couple of days' time, they will just be looking at the fixtures coming up."