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A particular common one is "it's not my fault." This is very popular, for example, with obese people who know they eat too much, but are nevertheless unwilling to change their eating habits. A popular it's-not-my-fault ploy in this case is to blame one's genes—I actually have seen this used explicitly in a food ad! Of course, if genes were to blame for the obesity epidemic hitting the Western countries (the US is not alone in this), we would have to assume that some mega-sized mutations happened over the last two generations—but people looking for an excuse will never question its underlying premises (another common ploy is to dismiss the simple arithmetic underlying the relation between calories burned and units of physical exercise as "useless" or "questionable" math).
More of my musings on this topic (I mean cognitive dissonance, not obese people!) in the comments...