Modeling an alternative flow of a use case as an extension of that use case.
Keywords: Addition to flow, alternative flow, extend relationship, extracting flow.
A clear sign of this modeling mistake is when the flow of an extension use case is supposed to replace a part of the base use case flow. Another indicator is, of course, that the base use case is not complete without the extension, because it lacks an alternative flow.
A use case has one flow, or sometimes multiple flows, considered to be the normal or basic flow of that use case. This flow will be ...