'a complete mistake' is correct and usable in written English.
You can use it to describe a mistake that is so serious, it is considered complete. For example, "Having a loaded gun at school is a complete mistake."
"I think it's a complete mistake and ought to be pointed out".
"It would be a complete mistake, because of the disappointment in Copenhagen, to abandon all our targets now".
"To mark down international stocks carte blanche because they're not U.S. companies was a complete mistake," he said.
I'm one of the ones who believed the Iraq was a complete mistake from the very beginning.
"It is a complete mistake to believe that there is a dilemma between schemes for increasing employment and schemes for balancing the Budget," Keynes wrote.
"This special clause was a complete mistake," said Yoshimi Watanabe, a member of Parliament who sits on the finance committee in the lower house.
complete mistake in the sense that everyone is worse off after it occurs.
Fireball is less of a whiskey and more of a complete and total mistake 98% of the time.
In his scenario, Jessica has just been waiting her whole life to be blessed with the attention of a complete stranger who mistakes hunched shoulders, darting eyes and rictus for laughing and smiling.
He added: "There was not just one mistake but a complete collapse, from top to bottom".
The structure of financial regulation in America still bears the mark of ideas forged in the Depression.Reform is certainly needed, yet, for all the excesses and instability of finance, a complete clampdown would be a mistake.
