In Varnishlog for http://meta.wikimedia.beta.wmflabs.org/wiki/Special:OAuthManageConsumers/proposed/1a31f6085491372db93394ed6cbff4c4 (and some other bugs that've since been fixed), this shows up:
30 ObjHeader c Content-Type: text/html; charset=utf-8 30 FetchError c Junk after gzip data 30 Gzip c u F - 20 0 80 80 90 30 VCL_call c error deliver 30 VCL_call c deliver deliver 30 TxProtocol c HTTP/1.1 30 TxStatus c 503
If you request the page directly from apache with curl, you get an error Call to undefined method Message::toJson()
Note that this particular example requires being logged in, probably as a (un?)privileged user
P633 also showed something similar happening on such errors.