If you can recollect more details, I would love to know what happeend, but I'm not sure about 3) I'm not aware of a pipelining-unsupported error in http (it is a thing in SMTP). It would take a very special HTTP server to look for another request in the socketbuffer after the current one and respond with failure.
I looked it up and I remembered incorrectly: the bug was due to the load balancer activating chunked transfer encoding to the backend nodes due to receiving the described HTTP2 request. It did not involve pipelining.
On the other hand, a quick search found evidence of some very special HTTP servers doing bizarre things with HTTP: https://github.com/elastic/elasticsearch/issues/2665