In America, this would likely qualify as fraud. According to Black's legal dictionary, fraud is defined as 'A knowing misrepresentation of the truth or concealment of a material fact to induce another to act to his or her detriment.' So, if someone did get away with it initially, it would be (hopefully) rectified by the court system eventually.

However, a quick google search found a case from Oklahoma in 2008 in which customers used disappearing ink to fill out checks. By the end of business, the checks were blank.

An especially interesting case I found occurred in France in 2011 in which the dealer would mark certain cards with invisible ink and his co-conspirators were equipped with special contact lenses.

Edit: I can't format apparently

Thanks! Yeah, I was thinking that someone could write a clause that would later vanish. I knew someone had to have at least attempted it.

Saw a show about spies once where it said they would mail what seem to be social letters to each other. Which would have secret messages and orders which were invisible until they were exposed to a solution. Some were activated by urine if the person was incarcerated and lost their belongings.

Thanks again!