As I noted in my earlier blog, the most worrying thing for Labour about the email by Miliband adviser Torsten Bell describing Ed Balls as a "nightmare" is that it was leaked at all (to the Mail on Sunday). If one or both sides have decided to go public with their disagreements, it doesn't bode well for party unity.
But I'm now told by a Labour source that the leak was an "unfortunate cock-up, rather than conspiracy". In his email on Balls's response to the Bank of England's upgraded growth forecasts, Bell intended to copy in Labour pollster James Morris but accidentally copied in Conservative MP James Morris instead (an episode worthy of The Thick Of It), who saw it fit to pass on the exchange to the Mail on Sunday's political editor Simon Walters. Fortunately for Miliband, this wasn't a red-on-red attack.
That's not to downplay the tensions between the Labour leader and Balls, but a deliberate email leak, reminiscent of the worst days of the Blair-Brown feud, would have marked a new level of hostility.