Flaw (noun)

1. A mark, blemish, imperfection, or defect which mars a thing's appearance, quality, or usefulness.
2. A weakness or error in a plan, system, or situation that can lead to failure or negative consequences.

Origin:

Middle english: from old norse flaga "slanting, oblique", of germanic origin; related to dutch vlak "flat" and german flach "flat".

Examples:

1. The diamond had a small flaw.
2. The painting had a flaw in the brushwork.
3. The system had a flaw that caused it to fail.
4. He found a flaw in her argument.
5. The constitution has a flaw that allows for abuse of power.