e.g. when I scanned http://musicbrainz.org/release/89c21e3d-f70a-493e-bafd-b5b20e06e556 (the barcode itself doesn't have the first zero, only the second digit and later), it didn't find that release despite the barcode (0-prefixed) being in the DB. Ideally, of course, the barcode is just entered correctly, but for these cases a 0-prefixed UPC is a valid EAN and should probably be searched.