Except that such things happen a lot more often on browsers. And "softer" errors, like experimental features being subtly different because user X toggled checkbox Y (which you don't know about and cannot reproduce) are not going to be caught by test suites.

They can be caught by client-side crash logging / reporting. But if they're useless, you just lost the ability to fix bugs you don't see yourself in your nice up-to-date dev environment. These bugs happen with surprising frequency in any reasonably-used site.