Final answer:
A crack is an imperfection that is considered unacceptable in materials regardless of size or location, as it can lead to structural failure.
Step-by-step explanation:
The imperfection normally considered as unacceptable regardless of size or location is a crack. While imperfections like porosity, inadequate penetration, lamination, and burn-through can sometimes be tolerated to a certain extent depending on factors such as size, location, and the standards applied to the work, cracks are typically seen as critical defects. These can lead to failure under stress and are not acceptable in structural components, especially in fields like aerospace, automotive, or construction where the integrity of materials is crucial.