Overview
404 errors can be costly.
Making an HTTP request and receiving a 404 (Not Found) error is expensive and degrades the user experience.
Particularly bad is when the link to an external JavaScript is wrong and the result is a 404. First, this download will block parallel downloads. Next the browser may try to parse the 404 response body as if it were JavaScript code, trying to find something usable in it.
Some sites have helpful 404 messages (for example, "Did you mean ...?"), which may assist the user, but server resources are still wasted.