There is no excuse. Stick to the manifesto or we should be able to fire them instantly and force another election.
Indeed, even if someone can win a majority, it's very bad liberalism to say they should be allowed to implement anything - politics is as much about fighting the tyranny of the majority as implementing what you want to.
Of course, when we're talking about the NHS changes, which were in no manifesto, but seemed to have been a tory plan for many years, we have a right to be angry. Indeed, the Lib Dem constitution has almost certainly been blatantly violated in respect of this and other policies - Our MPs aren't whippable on anything that isn't party policy, with the exception of the Coalition Agreement (which was passed by a vote at a special conference session, which I attended.)
Of course, they seem to accept the government whip on such matters, which makes activists like me very depressed for the state of our MPs (who are mostly the richest activists from their local area - since we're a very poor party who ask our candidates to fund their own campaigns - this is the reason I'm in favour of some sensible way of state funding parties, although I don't know I've ever heard a good implementation.)
[1] SNP. populist party without any real philosophy, thus able to seem to appeal to everyone simultaneously.
The Digital Economy Act was the only thing that has ever made me actually write to my (then) MP (Justine Greening) - the response displayed nothing but a complete lack of both care and understanding. Ridiculous.