Apparently, during the mvn release:perform, where the actual artifacts are uploaded to repository.apache.org, into an "open" staging repository, something hiccupped on my network connection, recovered, but it was then using a different IP. This caused repository.apache.org to create a new "session" for uploading.
As a result, the artifact upload was split between two staging repositories, and one of them ended up missing some required md5 / sha1 etc. files. The repository checks this when you try and close this, and makes the close fail if the files are missing. (You can drop it without closing, though). So, I'll retry building the rc3, but will call it rc4 now. -Marshall
