A flaw can be a sign of weakness or defect. If you try to make wings and fly off the roof but wind up crashing in the gutter, there's a flaw in your plan.
In the early 14th century, when the noun flaw was first recorded, it referred to a snowflake or spark of fire. That sense is now obsolete, and now we use flaw to describe shortcomings in either character or object. We all have flaws. It can also describe an intentional mark of imperfection. “You see,” said the dancer Martha Graham, “when weaving a blanket, an Indian woman leaves a flaw in the weaving of that blanket to let the soul out.”