"Major fault" refers to a serious mistake or error that has significant consequences. It implies a significant failure or flaw in something or someone. Full definition
An excessively narrow head is major fault in both types. (abra1st.com)
While some types of feet are desired by owners of pedigree dogs because they match the breed standard, other types are highly undesirable because they are considered major faults. (pethelpful.com)
It's smartly finished in the back, though, and few would otherwise find major fault with it. (autocar.co.uk)