Raith Rovers stunned the Glasgow giants with John Baird's extra-time winner to end Ibrox hopes of a treble.
The League One champions were overwhelming favourites to lift the trophy for the first time but put in an abject display as they lost 1-0.
To make matters worse, the chances they did create were wasted as Bilel Mohsni and Lee Wallace failed to convert during the 90 minutes while Nicky Law hit a post in extra time.
And Fifers' keeper Lee Robinson - the hero with former club Queen of the South in last year's final - was again a pivotal figure with a string of saves.
But having now failed to win any of the eight cup competitions they have entered during McCoist's reign, the manager is certain to come under fresh flak from fans.
However, the former Light Blues striker said: "I don't fear for my future at all.
"I know the fans won't be happy but no one will have to tell me anything, that's for sure."
McCoist - whose side face and even tougher test next Saturday when they take on Dundee United in the William Hill Scottish Cup semi-final - added: "I've had some big disappointments.
"That's up there with all the major disappointments that I've had, but it's not just about me - it's about the team and the club and the players, all of us together. So it's a big disappointment and we need to take it on the chin.
"We don't have time to feel sorry for ourselves. That's the one thing we can't do. We've got a big game to look forward to on Saturday."
Mohsni bookended the 90 minutes with two terrible misses when he twice somehow managed to head over from close range, while Robinson pulled off a terrific block after Fraser Aird crossed for Wallace at the back post just moments into the second period.
But it was Baird who wrote his name into Rovers folklore when he pounced two minutes from time.
Mohsni and his skipper Lee McCulloch should both have dealt with a loose ball but allowed Grieg Spence into shoot at Cammy Bell.
The keeper could only spill the ball at Baird's feet, though, and the former Partick striker - released in January - sparked wild Raith celebrations with a low finish.
With tears of pride running down his cheeks, the matchwinner - who secured the club's second ever trophy following their famous League Cup win of 1994 - said : "That was a big day in Rangers history today - and we were the ones to stop them.
"I don't think there are words to describe scoring a winner against one of the Old Firm in a cup final with a minute to go. It's just a dream.
"If someone had told me a week before the transfer window closed I'd be playing in a cup final against Rangers and scoring a winner I wouldn't have believed them.
"I was at a low because I wasn't playing football. So this is just unbelievable."
But for McCoist, he must now somehow pick up his shattered squad before next week's Ibrox meeting with United.
"I'm extremely disappointed, as are the players," he admitted. "We didn't play well but I thought we had the better chances. But if you don't take them you put yourself at risk and that's how it worked out.
"I've been here long enough to know that there will be a lot of unhappy people after this result. But none more so than the players and myself.
"But as low as we are, we can't stay down because we have a massive game coming up next week.
"We will need to play better than that because United are one of the form teams in the country."