If you are secretly, or not so secretly, plotting to succeed the prime minister as leader of your party, the one thing you don't want is the endorsement of the opposition.
Ed Miliband often complains that David Cameron asks questions rather than answers them during prime minister's questions, as that is currently his job, and today told him it was "good to see him preparing for opposition".
Of course if Cameron were to lose the next election the chances of him staying on as Tory leader are precisely zero. Picking up on the Westminster gossip this morning, Miliband anointed Theresa May, who was sat close to the prime minister, as Cameron's successor. "I'm looking forward to facing her in opposition," he said.
May's death stare told Miliband what she thought of that unwelcome encouragement. However the other cabinet minster said to be on maneuvers, Philip Hammond, appeared to enjoy the joke.
The Lib Dems, who have been remarkably loyal to Nick Clegg, also seized the chance to cause a little mischief. Cheltenham MP Martin Horwood said Lib Dem Mike Thornton's victory in Eastleigh showed "even governing parties can win marginal by-elections if they stick with their leader through thick and thin".
Cameron welcomed Thornton to the Commons and said he "noted very carefully" Horwood's reminder of how loyal Lib Dems are to their leader.