"drastic mistake" is correct and usable in written English.
You can use it to describe an error that has far-reaching consequences or serious implications. For example, "He made a drastic mistake when he entrusted his business partner with the confidential information."
Groups like the National League of Cities say that such cutbacks represent "a drastic mistake" for hard-pressed local communities.
Mr. Harkin said "it would be a terrible, drastic mistake" to freeze spending on labor, health and education programs, as Mr. DeLay had suggested last week.
Representative Dan Lungren, Republican of California, said the Judiciary Committee had made a "drastic mistake" by approving Mr. Schumer's proposal, which he called "an abomination". He said he feared that this provision was so generous to illegal aliens that it could "kill the bill" on the floor of the House.
"I think he made a drastic mistake," said Rutherford, predicting that the remarks would ultimately cost Paul votes in the region.
This would be a drastic mistake.
The City's top regulator last declared it was time to stop demonising overpaid bankers – some of whom he has previously branded "socially useless" – and focus on the drastic mistakes made by policymakers in the run-up to the financial crisis.
Always keep your cool, being over excited/scared/angry while in this type of situation can cause drastic mistakes that may either end up in you being injured, killed, or found out.
Her coach, Richard Callaghan, called the mistake "drastic" because it came in such an important competition.
He has not done anything drastic since, but there is no mistaking that he considers himself one of the team's leaders.
They also send the message that past mistakes can have the drastic consequence of permanently closing off one of the surest routes to a better future.
Make no mistake this disruption will be drastic.
