Manager Ronny Deila was furious after a blunder from officials in Celtic's extra-time Scottish Cup semi-final loss to Inverness Caledonian Thistle.
At 1-0 up, Celtic were denied a penalty when Josh Meekings stopped a Leigh Griffiths header with his arm.
"Everyone was angry. That's why it's hard to get the treble as small details can make you fail," said Deila.
"Everybody knows that the game would have been finished. Refs make mistakes, but this one is hard to take."
Celtic took an 18th-minute lead thanks to a curling Virgil van Dijk free-kick and enjoyed a period of dominance before the interval.
But they were denied the chance to extend their lead from the penalty spot on the stroke of half-time when Meekings's offence went unnoticed, despite the presence of an extra official behind the goal.
"I don't have to speak about that," said Deila. "People can see themselves and make a decision, but there were five referees today."
Greg Tansey levelled with a penalty after Celtic goalkeeper Craig Gordon was sent off for colliding with Marley Watkins.
Edward Ofere fired Inverness in front in the 96th minute before a blunder from goalkeeper Ryan Esson allowed John Guidetti's free-kick to bounce over him for the 10 men to equalise.
But, with the prospect of a penalty shootout looming, David Raven blasted home the winner from a tight angle to send the Highlanders into the final against Falkirk.
"We created big chances and should have scored more," said Deila, reflecting on the first half.
"The matched turned when we get 10 men. Before that, we had control.
"We fought really hard and we got tired and then we lost the ball too much.
"We were at least hoping we would get to penalties, but we conceded an easy goal in the last five minutes and we couldn't come back.
"Now we just have to give credit to Inverness and start thinking about an important game on Wednesday."
Celtic visit Dens Park to face Dundee aiming to extend their lead at the top of the Premiership to eight points going into the post-split fixtures.
"We have six games for the title and we need to be back on track," explained Deila, who had no issue with Gordon's red card.
"The league is always the most important thing.
"You get disappointment in football. You have to deal with failure. Today we were close, but we couldn't reach it."