SS3.0.3 rc1 although I think it was happening in 3.0.2 as well.
Seemingly at random times, a page in the CMS returns the error "Uncaught LogicException: GridField doesn't have a modelClassName, so it doesn't know the columns of this grid." The page throwing the error is using Page type - Page which is being used on many other pages throughout the site and those haven't exhibited any problems. So not sure why one is throwing the error and another is not.
So far the only remedy is to delete it via phpMyAdmin and then make a new one.
I've just encountered this issue, and it was because the page was the target of a RedirectorPage that had since been changed back to being just a plain Page.
Deleting the references in RedirectorPage and RedirectorPage_Live in the database solved the problem without having to recreate the pages in the CMS.
This looks like a bug in the way that pages are resolved in the back end, possibly just needing a check to see if a RedirectorPage entry is currently referenced by the SiteTree. I'll have a look to see if there's already a bug logged for this.
(Happening in 3.0.2 for me, by the way)
Edit: Looks like a known issue - http://open.silverstripe.org/ticket/7627 and http://open.silverstripe.org/ticket/8008