Handling of Expect: 100-Continue is partially broken.
After getting the Expect header, the server is allowed to:
1. respond with an HTTP 100 Continue status
2. respond with HTTP 417 Expectation Failed status
3. respond with the final HTTP answer, typically an error.
Handling of situation 1. seems to work ok. I haven't checked the scenario 2. But scenario 3. is broken, at least when using chunked transfer encoding.
8.2.2 Monitoring Connections for Error Status Messages
An HTTP/1.1 (or later) client sending a message-body SHOULD monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it SHOULD immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (section 3.6), a zero length chunk and empty trailer MAY be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client MUST close the connection.
The problem is that HttpClient does not send the last chunk in this case, nor terminates the connection, nor continues sending the body which are the only options allowed by the specs. Instead it just happily returns the response to the user and doesn't send anything to the server, keeping the connection open. This breaks subsequent requests on this connection, since a standard-compliant server would expect the request body and would interpret any subsequent HTTP status line as an entity chunk instead of a new request.
Debugging this is unfortunately quite hard, since many of the servers got this wrong either and they just close the connection in this case, which is a safe, but suboptimal default.