Last year I was travelling around Italy and somebody broke into the rental car I was using (a Smart Fortwo) and stole my luggage - which contained my camera, laptop and passport :D Although I had travel insurance and separate car rental insurance, neither covered theft of personal possessions from an unattended vehicle.

Even the 'super everything included' insurance rental companies try to sell wouldn't have covered this, as they exclude items accessible from the passenger compartment (like most small cars, the Smart Fortwo doesn't have a separate boot).