The problem is that References: headers are sometimes delivered as multiline headers by some clients. For example:

References: <msg-id>

  <msg-id>

  <msg-id>

  <msg-id>

  <msg-id>

A header can be continued to a newline if the first character is white space (tab or space)

When XOVER assembles the tab delimited list of headers it doesn't convert the CRLF or TABS to spaces as RFC 2980 calls for. In fairness it's an obscure footnote on a page other than the one the description of the format is on.