As soon as a conversation is marked long-running, it is assigned a cid. If the html response of the request that began the conversation contains links to resources (e.g. images) that include the cid in their urls, the browser may request these resources before the first request has finished.
Weld stores conversations in the cid-to-conversation map only at the end of a request. All requests that are received by the server before the first request has finished, will therefore receive a NonexistentConversationException.
Weld should be able to look up a conversation by id as soon as the conversation is assigned an id.