This error is definite mismatch between the data that is advertised in the HTTP Headers and the data transferred over the wire.

It could come from the following:

1. Server: If a server has a bug with certain modules that changes the content but don't update the content-length in the header or just doesn't work properly. It was the case for the Node HTTP Proxy at some point (see here)

2. Proxy: Any proxy between you and your server could be modifying the request and not update the content-length header.

As far as I know, I haven't see those problem in IIS but mostly with custom written code.

Let me know if that helps.