We have a webservice that is called from an application under heavy use.
From time to time we are getting errors coming back from the webservice:
The request failed with HTTP status 400: Bad Request
The stack trace is not very helpful:
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
So basically the webservice is just throwing back a 400 error to the webservice request.
I haven’t seen anyone online who has this type of issue. I have seen several where they ALWAYS get this error, but none where it works most of the time and fails sometimes.
I am considering that it could be something in the network, like a proxy server or AV sniffer, but not sure.
Chris May 
are you using MTOM? I am having (what I think is) the same problem. The same webmethod called by two threads at the same time (when requiring MTOM) one of the calls throws back a 400. Turn off MTOM and I don’t get the 400. Can’t find any info anywhere…