Example:

search for complete barcode w/ leading zero, returns a hit (as expected):
http://musicbrainz.org/search?query=barcode%3A055490182521&type=release&limit=25&method=advanced

replace a couple digits in the first search with a wildcard, returns no hits (unexpected, and incorrect):
http://musicbrainz.org/search?query=barcode%3A0554901*&type=release&limit=25&method=advanced

remove the leading zero from the second search, returns hits both with and without the leading zero (unexpected and a mix of correct/incorrect?):
http://musicbrainz.org/search?query=barcode%3A554901*&type=release&limit=25&method=advanced