See http://tickets.musicbrainz.org/browse/CAA-76?focusedCommentId=37055&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_37055 for an example. Quoting the response from there:

<?xml version='1.0' encoding='UTF-8'?>
<Error><Code>MalformedPOSTRequest</Code><Message>The body of your POST request is not well-formed multipart/form-data.</Message><Resource>signature 'jUnZxoP/XdnM4OR0j2ODWdKIYxk=' doesn't match computed: 'IJWx1j3lvkS9Wmicvuh5O1Sjq74=' for this policy</Resource><RequestId>b68b06cc-f95e-4654-bdf1-2029c55983ee</RequestId></Error>

Also reported at http://forums.musicbrainz.org/viewtopic.php?pid=31315#p31315