In the event, the proponent cites a general principle that is apparently uncontroversial, then illustrates how it applies to the present circumstances. The opponent is initially struck by this argument, as each leg of the syllogism seems impervious. Nevertheless he recovers quickly upon realizing that in another particular case, the proponent would not follow the principle as stated, i.e., is guilty of intellectual inconsistency.
When charged with this infraction, the proponent follows one of two courses. Either he explains that the second case is different, typically by elaborating and narrowing the scope of one of the terms in the statement of the original general principle; or he explicitly amends the general principle with one or more qualifiers. Both paths create the same result: they exclude the opponent's counterexample from application of the principle while preserving the original case.
The opponent is free to disagree with the updated and now less-general, more-debatable principle. However, as in negotiation anchoring, the force of the original general principle lingers and puts the him on the defensive as the discussion turns to the substantive questions at issue.
In such a scenario it is entirely possible that the proponent is in the right regarding the substantive debate. Whether or not intentional, however, the initial general principle acts as a manipulative rhetorical device. If the opponent is alert, he can return the argument to its rightful place in substance, possibly in a defensive posture; but if not, he may exit the scene with tail tucked.
To illustrate this process, I provide two examples. These particular cases were selected because, first, they are rough descriptions of actual conversations I have had with people; and second, so as to illustrate that this behavior is multi-partisan. Incidentally, I openly acknowledge the risk that I fall prey to a self-referential casuistry in this very selection of examples.
Those opposing the legality of abortion sometimes appeal to the "sanctity of life" as an argument. It is common, though not ecumenical, that these same individuals also favor capital punishment. When this apparent inconsistency is pointed out, the usual appeal is to add the qualifier "innocent." This reduces the universal appeal and power of the principle considerably, and relies heavily on the particulars of "innocent," including one's view of free will, the role of environment and genetics in shaping behavior, risks of epistemic error regarding innocence, etc. Interestingly, it does not seem to include the notion of original sin. In any case, the original proposal that the sanctity of life is key to the discussion is simply a distraction from the substantive question of when life must be respected and why.
Those who express anger at calls for the impeachment of President Obama sometimes refer to the principle that we should not "use the impeachment power frivolously." When one points out that similar calls came from the left for the impeachment of President G.W. Bush, these individuals respond that that is a different situation, it is not frivolous because (for example) Bush lied to the American people, as though Presidential prevarication were extraordinary. This returns us to the substantive matter, which is precisely the question of whether impeachment is actually appropriate.
When presented intentionally, casuistic arguments are at once cynical and hypocritical. Intellectual inconsistency intentionally applied is very nearly the definition of hypocrisy. Such techniques are cynical because they degrade the value of following general principles merely to win an argument or one's preferences. I urge the reader to refrain from casuistry, and to fiercely resist when facing it.