dev-java/json from java-overlay is currently at 1.0. Unfortunately, the SRC_URI doesn't contain any version info (http://www.json.org/java/json.zip). It seems the file was recently updated, because the recorded filesize in the digest doesn't match anymore. Maybe a bump to 1.0-r1 or 1.1 or similar would be helpful.
I datestamped and mirrored the sources.