With conceited good intentions, it is about “Look how wonderful I am for having helped you”. With genuine good intentions, by contrast, the accent is on you — and not that I have helped you...
When I think of white liberals, I am stunned by how interested they (initially) are in helping me and how much they admire me just so long as I underwrite their image of a white who just adores people of color.
Most significantly, I am stunned by how annoyed most white liberals are when it becomes manifestly clear that I can do rather well on my own.
Another case: I've always been bothered by the right-wing argument that private charity is morally superior to government aid. Surely the policy focus should be on helping those in need, not providing rich people with opportunities to be "virtuous"! (It's like pushing the Lifeguard aside so you can be the one to save the drowning victim. Ugh.)
Finally, for a more commonplace kind of example, see Publius' post at Obsidian Wings, about how self-described "Moderates" can be manipulated by partisan ideologues who exaggerate their position in order to shift the middle ground in their direction. I'm more sympathetic to this case: I can feel the attraction of bipartisan moderation as an ideal, and the heuristic of locating the truth in "the middle ground" may work more often than not. Still, heuristics are fallible, and in this case open to abuse. Moderation is itself an ideology, and no substitute for the hard work of honest inquiry and critical discernment.
It's easy enough to pick the position that looks, superficially, like what a reasonable person would endorse. Suggest a compromise, applaud the middle-ground (but not too loudly!), and you'll look good. These actions support a self-image that resonates with your vision of virtue. The only problem is that appearances can be deceiving. A deeper level of moral seriousness would require us to actually be that reasonable person, i.e. actively exercise our faculty of reason in assessing the first-order issues on their merits.
This gets better results. Further, it avoids the vice of self-idolatry. We find that the actual exercise of virtue involves an outward focus. You support X on its merits (due to appreciating its right-making features), not just because it reflects your desired self-image.
So, it seems, there's a sense in which we shouldn't fundamentally aim to be ideal agents after all. We should instead conceive of virtuous character as a mere means, a kind of guiding ideal that will help us realize true value in the external world (which is what really matters). This conception will hopefully make us less susceptible to 'image' hang-ups. Sure, we should all try to be better people, but it isn't the goal -- it's merely a first step.