couch_replicator_httpc's retry mechanism interacts poorly with the multipart requests we use to transfer attachments during replication. In the case of PUTs, the replicator will retry a request even though it has already streamed part of the request body. For GETs, it can potentially interleave body data from the original response and the retried one.
I put together a fix at https://github.com/cloudant/couch_replicator/pull/36 that disables the default httpc client retry mechanism for multipart requests and manages the retry logic "atomically"; i.e., it retries the original GET request if the GET or PUT fails, and it ensures that the original GET request is properly terminated during the retry. I'll push a version of that patch to the Apache repo now.