Our build shipped clean and still broke
Building a site for a supplement brand right now. Product pages had a flavor picker and an image gallery, both looked finished. Then one script had a brace in the wrong spot. Nothing threw an error, nothing broke loudly. The gallery just sat blank and the picker buttons did nothing when you clicked them.
We only caught it by clicking through the actual page like a customer would, after every check upstream said it shipped clean.
If someone builds you a website or sets up an automation and tells you it's done, that means it ran without an error. It does not mean it works. Go click every button and load every page yourself before you trust it.