colossal blunder

By focusing on just one part, and assuming it's the whole, you make a colossal blunder.

He vigorously condemned the Crimean War, and took his stand with the few, who tried to save England from that colossal blunder.

The most dangerous experiment in modern times in government had proved to be a most colossal blunder.

The knowledge that he had made the one Class A colossal blunder of his thirty years in politics, this last half-day.

To ignore their claims, politically, he contended in and out of print, was a colossal blunder.

M. Gerbois, in his exasperation, was guilty of the colossal blunder of showing this letter and allowing it to be copied.