When is a mistake of law a valid defense?

This is usually phrased as "ignorance of the law is no defense." Just because you're not aware something is illegal doesn't necessarily mean you can't be charged with the crime of doing it. That being said, there are a couple situations where ignorance is an excuse. The first is when the law you're being accused of breaking was not publicly available (after all, if you never had a chance to know the rule, its not fair to punish you for breaking it). Note that in these days, when laws are available for review online and through numerous resources, this exception is pretty hard to rely on. The second is when a person reasonably relies on an official statement from an official source about the law that is actually inaccurate. It'd be like if the Governor went on TV and said, "Hey everyone, tomorrow we are pausing all parking tickets, so feel free to park anywhere you want." It wouldn't be fair to have a bunch of people rely on that statement, get a parking ticket, and then say, "Well, wait a minute — I was told this was okay!"