Steps to reproduce: just try to upload something.
On beta you will get an 503 (not sure if that's normal or a secondary bug), locally you will get a fatal.
Mark got a more useful error using Zend:
Fatal error: Cannot use object of type LocalRepo as array in .../mediawiki/includes/filerepo/FileRepo.php on line 1283
Presumably related to I015635a9bf080ef6d98b2cff49b949c4378a859f.