An Unforced Error

In tennis, an "unforced error" is an error caused purely by one's own inability to succeed. It is a failure that cannot be blamed on the opponent or otherwise excused.

In business, and as an idiom in general, an unforced error is a unilateral failure, a problem that was caused solely by one's own side.

Example: Recently, hacking (of some sort) has been responsible for a giant breach of the Sony Playstation Network, which has some 70 million customers, many of whom could have credit card data that could (theoretically) be stolen. In contrast, an employee of an American baseball team, the New York Yankees, accidentally e-mailed the personal information of some 17,000 season-ticket holders to hundreds of people.

Of these two situations, the first was caused by an outside party; the second was an unforced error, purely the fault of the employee in question. Sony may have a case that it did everything reasonable to prevent an attack on its system; the New York Yankees team does not.