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We noticed today that when we typed in the domain.com for our site, rather than www.domain.com, using redirection based on url, it showed the mobile version of the site on desktops.
In our case, we corrected it by adding a redirect on our server to redirect domain.com to www.domain.com, since that was something useful anyway.
Seems like, as long as a subdomain is entered, things work as expected, but if the site visitor leaves it off, hilarity can ensue.
Maybe something buried in one of those pregs comparisons?