Yes politicians will sometimes break their promises or for some reason not be able to fulfil them. When the Liberal Democrats made their manifesto they did not expect to be in coalition with the Conservatives, with two incompatible manifestos some things were going to have to be dropped. Equally sometimes the party in power will find they can’t get through the changes they want. The point of voting when events might overtake a manifesto is that the party’s ideology will tell you how they are likely to react – a libertarian in 10 Downing Street would have let the banks go bust or a communist would have nationalised them. Many could have anticipated that a Labour government would engage in some kind of bail out to save savers and the system. By having voted for the Labour party voters were saying they wanted a slightly left of centre response to events.