Having witnessed the original feud at first hand, both men are conscious of the need to avoid an irrevocable split.
Miliband's adviser Torsten Bell accidentally copied in Conservative MP James Morris, rather than the Labour pollster of the same name.
The key point about the email by a Miliband adviser describing Balls as "a nightmare" is that it was leaked in the first place.
Plus: The last word (for a while) on Ed Miliband's appearance.
Promising an in/out vote would shift the debate back onto Tory territory and allow Cameron to claim that Miliband is dancing to his tune.
MPs who last year wanted the Labour leader to trump Cameron by calling for a snap poll on Europe say the moment has passed.
In May 2010, Cameron declared that the living wage was "an idea whose time has come". But the PM has said nothing substantive on the subject since.
The challenges facing the Conservatives are mostly structural and may be impossible to overcome.
"The first and last test of economic policy is whether living standards for ordinary families are rising."
On the politically defining issue of low pay, the Labour leader has the pitch all to himself.