I did not dig too much into this case, but I guess it has to do with imaptest stressing the hell out of it and deleting/expunging a given item at the same time that a sequence was queued for a fetch. In any case, we need to defend against these errors, or we'll end up with a None passed to the spew_foo methods instead of a well-formed IMessagePart, which will throw much nastier errrors... and block the reactor by the way.