under heavy load i see multipart message coming in intermixed.
this happens with a XREP/XREQ combination.
i reduced this to a testcase:
there is a server.cc and a stress.cc - each part of a multipart
message is prefixed when sending. and the receiving side checks that
all parts of the received mp-message have the same prefix. which fails.
there is also a output.txt where you can see how message-parts come in.
every line is prefixed with the hex-representation of the identity
received on the XREP side (first part).
this happens on 2.1.4 and on 2.1 branch in git from yesterday.