This bug is based on the issue I reported via email:

It appears that if I cancel a job, Client::OnDownloadSucceeded can still be called. 

This happens because that call is posted, rather than invoked first so presumably the download completed before I cancelled it (but my code had no way of knowing that).

Additionally I think it's possible for the client to receive two OnDownloadFailed messages if they cancel. One with CANCELLED and one with something else.

It seems to me that methods like SendOnDownloadSucceeded (which are posted), should check if the download has been aborted before actually sending the message.