static $allowed_children = array('Newspage'); // if newspage is not a class -that is a php file in mysite- you get an error in the cms backend, nowhere else.
I'm not sure if people can reproduce it since it did not show up immediately, but removing all the php files (besides page.php) in mysite/code and putting them back in one by one + dev/build brought back the error and the error was gone after I corrected the classname.
Maybe someone else with this error can use it as a pointer...
Working with MAMP on osx 10.6 with Silverstripe 3.01 and 3.02