Connection reset by peer (502) when contacting WCF service in IIS 7.5

We have a WCF service that has been working fine for months. Just this morning, calls to this service began failing with the proxy server reporting “connection reset by peer” (502). Oddly enough there are no IIS log entries for the dropped connections, although we are able to detect them with WireShark. So it seems as if the thread is dying, leaving no trace behind.