jdMorgan - 12:30 pm on Oct 27, 2010 (gmt 0)
The HTTP protocol document that I linked to above should be treated as "the law." In some cases, your site can break the law and get away with it. In other cases, breaking the law will result in your site getting the death penalty. So, it is not good to be ignorant of the law or to break it.
HTTP requires that certain server responses occur under certain circumstances, and these responses form the very foundation of how the Web works. If you return a 301, 302, 303, or 200 response when a 404 or 410 is called for, you should expect to have serious problems with both users and with search engines.
Returning incorrect server responses is no more safe than driving the wrong way on a high-speed road... and usually has a similarly-unpleasant result.