It's feasible there could be a race condition here, but I'm not really seeing it and there is code in multiple places to prevent it. There's some client side code which would prevent drafts being saved while a submit is pending. There's also the code that should delete the draft once it has been submitted.
I can only assume this can happen if there's some sort of event on the network or on the server that prevents the draft from being cleared. There's no reason this would only affect conversations either, so it's possible we're dealing with something uber rare here but you just so happen to have both come across it.
Let us know if it continues with some regularity.