After asking this question: https://stackoverflow.com/questions/8042748/facebook-login-returns-error-message-if-domain-accessed-without-www-prefixI was told that using domain.com and www.domain.com at the same time is wrong and, to cite the answer You will end up with all sorts of problems.What kind of problems are those? The thing is the new company I work for does the same mistake and I'd like to explain them it's wrong and why but for now facebook login (which they don't use) and google analytics statistics mess I had to fix (4k selfreferrals a day) are the only reasons I know.
why is having no redirect between domain.com and www.domain.com wrong?
redirects
The answer when you asked on your original question was:Cookies are split and/or duplicated. SSL doesn't work. Inbound links  aren't consistentThe main thing to understand is that although www.domain.com and domain.com may display the same site, they don't have to, so are treated as separate sites.In detail Cookies are split and/or duplicatedYour users will end up with cookies for the domain.com and www.domain.com which may be different, depending on what you store in them. So, for example, if your website displays different content according to what's in the cookie then they'll see different things according to whether they're using www.domain.com or domain.com to access your site.SSL doesn't work.I think they mean, if you have an SSL certificate for domain.com then if people use www.domain.com then that certificate won't work (you'll have to get another one, or pay more for a wildcard one).Inbound links aren't consistent.Links could put to either version, so diluting the effect of the links - it looks like people are linking to two different sites. You could also be penalised for having duplicate content on two different sites.