A few days ago, a colleague asked me about why he couldn’t access the page with preview mode, and I realized that when he was previewing the page, by default Sitecore is using the anonymous user account.  It also described here .   Apparently the bug has been resolved in Sitecore 7.2+  and to resolve this issue the new settings “Preview.AsAnonymous” has been introduced, the default value is true:

<setting name="Preview.AsAnonymous" value="true" />

So, if the value is true, Sitecore uses the Anonymous user when one previews an item, otherwise, Sitecore uses the current user account.