The Blunder reference article from the English Wikipedia on 24-Apr-2004
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Blunder

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A blunder is a spectacularly bad decision with detrimental consequences to the party that makes it, the result of not reading signs, or misinterpreting available information. By contrast, many sensible decisions may prove disastrous in retrospect. The term is often used to refer to military, diplomatic, political, social or business decisions. The word comes from the Old Norse blundra "shut one's eyes" in the oldest sense in Middle English, "to stumble around blindly" all from a presumed an Indo-European base *bhlendh- that also gave us "blind." This modern sense in Wikipedia, dates from 1711.

A less consequential blunder is a faux pas.

With its background in "blindness" a blunder must be seen in the context of what would have been considered possible at the time.

Examples of blunders include:

Wikipedians are invited to come up with better, more specific blunders.

In chess, a blunder is a very bad move, often given the '?' or even '??' sign. But what is a blunder, also depends on the player, since a lesser move for a club player may be called a blunder if a Grandmaster plays it.

In science, some blunders may turn out to have positive consequences. Einstein called his introduction of the so called cosmological constant his "biggest blunder" of his life and abandonded the idea. Nowadays this constant is needed to explain the increasing rate of the expanding universe.