Scot Christie, who had failed to finish at either of her previous 500m and 1500m disciplines, sought to force her way through for a top-two finish.
But she tangled with China's Jianrou Li and both were disqualified for impeding, leaving race winner Suk Hee Shim and another Chinese skater, Kexin Fan, to go through.
The skater from Livingston had shrugged off her double disappointment by easing through her quarter-final in first place.
Christie started on the inside lane but suffered a minor knock on the first bend and settled for skating round in fourth and last position.
However, on the final lap Christie pounced, and when rivals Marie-Eve Drolet and Veronique Pierron tangled, the Livingston skater jumped to the front.
Giving her assessment of what had happened in the semi-final, Christie said: "When I saw the draw come out I saw it was the harder heat of the two - it was more like a final would be normally, and it was lane three, which isn't of benefit to me.
"So I thought I'd just save my energy and try to smash it at the end, and I knew if I kept patient I had the speed, so that is what I went to do.
"When I was coming around the outside, I saw they went wide because they knew I was coming. So that is why I switched inside and then pulled tight early, and thought 'I've done it'.
"I didn't need to go for the win - I thought 'I've qualified, that's what I need, and I need to save my energy for the final.'
"But unfortunately, I got knocked from behind unexpectedly and fell over but I still thought it would be fine. And then I got the penalty."
Asked if she knew exactly what the penalty was for, the 23-year-old said: "Vaguely - I've not talked it through with them but he (the referee) said I pushed her over."
She added: "The problem with short track is that the referees are always different and there is no kind of consistency because every referee has a different opinion.
"I always respect the referee's final decision, and I have to accept it anyway because that is short track, but I don't agree with it."