Donny and his sibs were probably right about one bad apple not spoiling the whole bunch, girl. But on the Web, users make snap judgments — so one mistake, and can throw the whole site into question. (Kinda like a spelling error on a resume: probably doesn’t make you any worse at driving snow plows, but the HR folks will toss you in the no-thanks pile anyway.)
To site visitors, many things can trigger this judgment: broken links, missing images, poor design, dated content, spelling errors.
I guess site visitors are looking for clues that help them measure esoteric values like “quality” or “validity.” When they see a mistake, they extrapolate. That can be very dangerous.