Some form of error that originated in the design phase and continued to survive into the implementation. Typically something that was simply overlooked, or the thing in question hadn't been meant to survive in whatever environment it finds itself in. A good example of this is the story at A design flaw in goldfish.

Other notable design flaws include the Tacoma Narrows Bridge, passenger-side airbags (when they kill people/children), and of course the Titanic. The software industry sees this form of problem every day. No jokes about Microsoft, please.