In the run-up to the 2010 election, they promised to oppose any rise in tuition fees, then reneged on this in government.
On its own, that's probably not that significant, but it appears to have helped cement their reputation as untrustworthy, not least among those who thought they were voting for a party more left-wing than Labour have become in recent years.
But... they came last out of the three main parties, so it is hardly surprising that they weren't able to implement the entire of their manifesto. The conservatives reneged on many of their manifesto pledges too.
I suspect it is more that the conservatives have much stronger support by the media.
It wasn't that we (I was a Lib Dem candidate in the election) promised it in the manifesto; it was that individual MPs made a personal promise to vote against an increase in tuition fees, and then voted for it.
On its own, that's probably not that significant, but it appears to have helped cement their reputation as untrustworthy, not least among those who thought they were voting for a party more left-wing than Labour have become in recent years.