Moral trump card
Regardless of whether they admit it or not, any system of ethics has an implied ethical calculus.
"Happiness and moral duty are inseparably connected." -George Washington
Anything considered an absolute moral good or evil within an ethical system, where it is unacceptable under any circumstances, can be considered to have infinite moral value in the utilitarian ethical calculus.
Bad Math
The presence of infinity really messes with your equations, as anyone familiar with math knows. In reality there are no infinities in utilitarianism because any action can only have a finite impact on human happiness. Except maybe blowing up the universe. Destroying the entire universe would be infinitely bad. Everything else is finite.
Bad Utilitarianism
Deontology is all moral trump cards, which is a good argument for utilitarianism. Human rights and other concepts that appear to contradict utilitarianism simply have very high intrinsic moral value. The actual value is much higher than utilitarianism's detractors assume it is, but its' still not infinity.
In the quality of life versus quantity of life moral quandary, the notion of "thou shalt not kill" is the quintessential moral trump card. If the fetus counts as a human life, then abortion is murder, and you can't beat murder so we take the trick.
Bad Outcomes
These moral absolutes often lead to morally outrageous outcomes while bolstering the believer with a false sense of righteousness. By playing the trump card, they appear to be taking a principled moral stance, when they actually just lack nuance.
For more card-based metaphors see The Gambler or listen to the songs on this page.
Card Playing Songs
You can tell the moral trump card by the way she shines.