The software behind the site falls a version behind, then three, because every update carries a small risk of breaking something and there is never a good week for that. A form stops sending and nobody finds out until a customer mentions it, which means the customers who didn’t mention it went elsewhere. Prices drift away from what things actually cost. The backup everyone assumes is happening has not been tested since it was set up, which is another way of saying nobody knows whether it works.
None of these announce themselves. There is no error message for “this page has been wrong since March”. A website with a problem looks exactly like a website without one, right up until the difference shows up in the figures.