In the recent dispute about means and ends on Catholic and Enjoying It!, a few people advocated a position that might be paraphrased thusly:
People who say the end doesn't justify the means just play around with the definition of the means until they can justify the end they desire.
You understand how the process works. You can work your way down the chain of intent: "Getting drunk is sinful? But I'm not getting drunk, I'm celebrating the New Year." Or the other direction: "But I'm not getting drunk, I'm pouring a clear liquid into my mouth and swallowing."
It wasn't clear to me at the time what the point of advocating that position was. Do people try to rationalize their sins? Certainly. Does this invalidate the moral principle that the end does not justify the means? Certainly not. If the advocates were merely trying to impugn the moral honesty of those they were arguing with, then it's no big deal for discussion; I personally can't claim to have never done some weaselly ratiocination to excuse something appalling I had done or was going to do. It may also have been the last, bent dart in the quiver, thrown into the discussion out of sheer cussedness. Sheer cussedness don't confront me.
If, however, it was meant as an attack on the principle itself -- if it was intended to suggest that the principle can't be properly applied in practice -- then we've got an issue.
We would all agree, I trust, that misuse does not take away use. (Good thing, too, since there is pretty much nothing humans are incapable of misusing.) Pointing out that people sometimes or even often misuse a moral principle is only an attack on the principle itself if the implication is that people always misuse it, or at least that there's no objective way of verifying that in a particular case the principle was used correctly.
But in fact there is an objective way of verifying that in a particular case the principle was used correctly. You just start with the end, with the answer to why you're doing something, then work your way backward along the chain of intent until you reach a purely natural (or physical or pre-moral) act, an act that has no associated moral choice. Since the end does not justify the means, and we intend the means we choose, every link in this chain has to be morally good.
Each link can be seen, for the purposes of analysis, as the object of an act the intent of which is the subsequent link. As the blessed John Paul II put it in Veritatis Splendor 73, the question is "whether that object is capable or not of being ordered to God, to the One who 'alone is good,' and thus brings about the perfection of the person."
Now, figuring out whether an object is capable of being ordered to God can be a tricky business, and it may be done more or less correctly in a particular case by a particular person. But it is not impossible; the perfection of the person is not a wholly opaque state. There is a perfect Person sacramentally present in your neighborhood Catholic church, if you want to go check your answer with Him.