Using an expression filter to set a header field with a value that contains newlines results in an invalid http request. An example is setting the SSL_CLIENT_CERT as in:

{{ <expression-filter name="ClientCertHeader" }}
  expression="set(attribute='%{i,SSL_CLIENT_CERT}',
  value='%{SSL_CLIENT_CERT}')"/>

The resulting http request doesn't include the line continuation sequence '\n\t' or '\n '. In the case of a reverse proxy the request is rejected by the proxied server.