A blunder is a spectacularly bad decision or action, a mistake or error with detrimental consequences to the party that makes it. It is typically attributable to faulty perception: the result of not reading signs, or misinterpreting available information. Naturally many sensible decisions, which even in retrospect were carefully taken, may also prove disastrous mistakes.
The term blunder 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 is dated from 1711.
Examples of actions famously considered to be blunders include: the Japanese attack on Pearl Harbor, the Maginot Line, and The Tea Act of 1773 (and related British policy decisions toward the American Colonies). However, there is often considerable debate as to whether a decision leading to failure is truly a "blunder" or merely a reasonable course of action based on the available knowledge at the time. Hindsight usually allows one to see the situation far more clearly.
A less consequential blunder is a faux pas, or a blooper.
In Go, there is a slightly more nuanced Japanese language term poka, meaning an unworthy slip by a top-level player. Fujisawa Hideyuki, in terms of pure talent one of the greatest go players, was famous for poka.