It's an irresistible opportunity for the new crowd to show that they are not afraid of making "the tough decisions" - or some such equally fatuous sound bite - and the outcome is inevitably an unplanned expense that skews the budget, sinks the manifesto and makes for unwelcome headlines.
You get the feeling that something of this ilk happened in Caerphilly where the chief executive has received a tidy pay rise since May for services yet to be rendered. The suspicion is that the Plaid opposition were laying in wait for the Labour newbies to drop this particular bollock as it was something they had themselves rejected when in office.
There was a similar experience in Swansea a few years back which saw a new Lib Dem controlled cabinet sign up to a previously scoffed-at scheme that eventually transformed the city centre road system into a three-dimensional scrabble board. The rest, just like Swansea's growth prospects, is history.
What's funny though is that the same individual was a key player in both instances.