You have made a major mistake.

你犯了一个很大的错误。

youdao

I made at least one major mistake of commission and several lesser ones that also hurt.

我至少犯一个重大错误，还有一些错误不那么严重，但也造成了不良后果。

youdao

Another major mistake that many people make is trying to fit in too much text on any single slide.

很多人犯的另一个主要错误是往每一页幻灯片上填太多的文字。

youdao

Russian officials, including Yeltsin, have warned that NATO expansion at this time would be a major mistake.

CNN: Clinton, Yeltsin Meet In Finland

The only way any competitor has a chance, is if Facebook makes some kind of major mistake.

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He wrote to James Madison that the omission of a Bill of Rights would be a major mistake.

FORBES: The Ninth Amendment: The Value of Our Unenumerated Rights