A mathematical puzzle for you: when does zero not equal zero? Today's answer: when reporting the amount of trans fats contained in the crappola food we oftentimes eat.
As of 2003, food companies are required to list the amount of trans fat contained in their product. However, this measurement is reported in grams, which has caused the ever-so-brilliant government to create a lovely little loophole for the food industry. A product can have up to 0.4 grams of trans fat per serving and still report it as 0g. Bloody brilliant! So, the food industry has to report presence of trans fat, except when it doesn't want to do so, and can then play the old serving size game to reduce the serving size until the amount of trans fat can drop to 0 on the nutrition info label, and then life is swell (for them, not us).
For more information, check out the Ban Trans Fats campaign. Full information there on this labeling insanity as well as extensive information on why this kind of fatty acid is bad for you.