Sporadic 502 error with gwt rpc calls - gwt

Question

I have a GWT application that is giving sporadic 502 errors all of a sudden. I have managed to replicate it by opening multiple connections to the application. Eventually I get a 502 error and the response headers for look as follows:
Server: squid/2.6.STABLE5
Date: Fri, 19 Aug 2011 12:08:03 GMT
Content-Type: text/html
Content-Length: 1014
Expires: Fri, 19 Aug 2011 12:08:03 GMT
X-Squid-Error: ERR_ZERO_SIZE_OBJECT 0
X-Cache: MISS from sentinel.bsgza.bsg.co.za
X-Cache-Lookup: MISS from sentinel.bsgza.bsg.co.za:3128
Via: 1.0 sentinel.bsgza.bsg.co.za:3128 (squid/2.6.STABLE5)
Connection: close
The response headers for the successful rpc calls look like this:
Date: Fri, 19 Aug 2011 13:04:37 GMT
Server: Apache/2.2.14 (Ubuntu)
Content-Encoding: gzip
Content-Disposition: attachment
Content-Length: 249
Content-Type: application/json;charset=utf-8
X-Cache: MISS from sentinel.bsgza.bsg.co.za
X-Cache-Lookup: MISS from sentinel.bsgza.bsg.co.za:3128
Via: 1.0 sentinel.bsgza.bsg.co.za:3128 (squid/2.6.STABLE5)
Connection: keep-alive
We have been able to repeat this on a local server too so it is not a network issue

Answer 1

Try not to route your RPC call via a proxy (Squid). Or at least try to configure Squid to not try to cache them, but only to forward.
Update
It's suggested here that such condition might occur with HTTP POST (used by GWT-RPC) by clients behind PPPoA gateways (cable modems) which have wrong MTU set. Do you see this errors from such clients?