Examples: A part gets obsoleted by vendor, sustaining engineering has to find a replacement. They conduct a small study to see how the new part works. The budget for conducting lengthy tests to determine if a simple change has any effect at the worst case tolerances is typically not there.
Suppliers quality changes over time. This is known as supplier quality fade. Suppliers will often underbid a part to win a contract. Then once volume ramps up they begin value engineering. Maybe they add more regrind material to an injection mold. Or they progressively begin to make the part slightly thinner on each subsequent batch. Maybe they begin to use a toxic filler material without notice.
An colleague literally had a situation in which a steel casting supplier was putting rocks in the castings.
Companies move injection mold tooling from vendor A to vendor B. The tooling might be the same but the settings are different, so the results are then different.
Without an excellent quality team that has money and time to check everything this will happen.
If you are competing in a marketplace with low margins there will never be enough money to check everything.