Final answer:
Ex post facto laws are laws that make an action illegal after it has already been taken. The big problem with ex post facto laws is that they violate the principle of fair notice and can lead to unfair and unjust treatment of individuals.
Step-by-step explanation:
Ex post facto laws are laws that make an action illegal after it has already been taken. They have a retroactive effect, meaning they can be used to punish an act that was not a crime at the time it was committed or to increase the severity of punishment after the fact. The big problem with ex post facto laws is that they violate the principle of fair notice and can lead to unfair and unjust treatment of individuals.