Here's a tolerable law review topic. One which I'll illustrate with the following hypothetical, which is an extreme application of the principle:
In 1993, the rules in murder cases allow introduction of X, Y and Z into evidence to prove an offense. In 1994, Flores allegedly commits a murder and flees to Mexico. In 1995, the Legislature changes the evidentiary rules, holding that not only are X, Y and Z admissible, but so are A through T -- all of which are things that are only tangentially relevant to the crime, but which allow the prosecution to "dirty up" the reputation of any defendant and thereby make a conviction easier. Flores in captured and prosecuted in 2006. Would the introduction of A through T violate the Ex Post Facto Clause?
What do you think as a matter of first principles? Would your analysis be altered given the classic exposition of the Ex Post Facto Clause in Calder v. Bull (which was reiterated by the Supreme Court in 2000), in which Justice Chase -- way back in 1798 -- said:
"I will state what laws I consider ex post facto laws, within the words and the intent of the prohibition. 1st. Every law that makes an action done before the passing of the law, and which was innocent when done, criminal; and punishes such action. 2d. Every law that aggravates a crime, or makes it greater than it was, when committed. 3d. Every law that changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed. 4th. Every law that alters the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offence, in order to convict the offender."
Hmmm. Whatcha think?
I'll give you the answer at least according to the California Court of Appeal. No Ex Post Facto Clause violation in our hypothetical. Read the opinion to see why (and whether you agree).