"significant mistakes" is a correct and usable phrase in written English.
Example sentence: He made several significant mistakes that cost him the game.
Significant mistakes will be exposed by the experiments of others.
Even the most successful candidate inevitably makes significant mistakes.
A number of significant mistakes need to be recognised and rectified.
Dr. Hauser has already acknowledged making "significant mistakes," but has admitted to nothing worse.
"The decision will go down in history as one of the most significant mistakes that Court has ever made," he said.
A year later, the big-money signings of Colorado's Mike Hampton and Denny Neagle and the Dodgers' Darren Dreifort are all considered to be significant mistakes.
"They felt like it was a significant mistake on my part.
Woods played steady through Amen Corner, the only player to that point without a significant mistake.
He acknowledges that the administration "made a significant mistake in not consulting adequately either Congress or its allies".
Her fall in prelims was actually her first significant mistake of this comeback, a momentary throwback to 2011 when Douglas had been considered an iffy competitor.
Google said adopting either notion would be a "significant mistake, as a matter of public policy and as a matter of fairness".
