The courts accept good faith arguments at times. They will give reduced sentences or even none at all if they think you acted in good faith. There are enough situations where it is legal to kill someone that there are laws to make it clear that is a legal situation where one person can kill another (hopefully they never apply to you).

Note that this case is not about ignorance of the law. This is I knew the law and was trying to follow it - I just honestly thought it didn't apply because of some tricky situation that isn't 100% clear.

The difference between "I don't know" and "I thought it worked like this" is purely a matter of degrees of ignorance. It sounds like the cops were ignorant of the law in the same way as someone who is completely unaware of it, just to a lesser degree. Unless they were misinformed about the origins of what they were looking at, it doesn't seem like it would be a matter of good faith, but purely negligence.

There was a circuit split and a matter of first impression in this circuit.

“Mens rea” is a key component of most crimes. Some crimes can only be committed if the perpetrator knows they are doing something wrong. For example, fraud or libel.

> “Mens rea” is a key component of most crimes. Some crimes can only be committed if the perpetrator knows they are doing something wrong. For example, fraud or libel.

We're talking about orthogonal issues.

Mens rea applies to whether the person performs the act on purpose. Not whether they were aware that the act was illegal.

Let's use fraud as an example since you brought it up.

If I bought an item from someone and used counterfeit money on purpose, that would be fraud. Even if I truly believed that doing so was legal. But it wouldn't be fraud if I didn't know that the money was counterfeit.