It was also a source of obscure bugs and DoS-level resource usage in servers and (non-browser) clients. Some servers managed to send multiple responses together

It was also a source of obscure bugs and DoS-level resource usage in servers and (non-browser) clients. Some servers managed to send multiple responses together or deadlocked the connection when the timing of requests or available responses was just right. Some servers would queue up responses in memory and so were prone to excess resource usage. Also, if you had a sent request waiting behind a slow/large response, you had to decide whether or not to send an extra request on a separate connection to get the resource sooner. And when a server closed a connection after sending a response (the spec allows it for multiple reasons), it was hard to know whether it was that the server was just not accepting more requests on that connection or whether a particular request caused it and hence whether any of the waiting request(s) should be re-sent or treated as being in error.