Can I get arrested for unknowingly cashing a bad check?
Yes, even though you say it was "unknowingly," if there is evidence you knew, or at least reasonably suspected, that the check would bounce. But if you had no idea the check was bad, you had no criminal intent.
In criminal law, intention is everything. What separates a non-crime from a criminal act is generally the intent of the person acting. For example: you and I have identical suitcases and I accidently pick up your suitcase from the airport luggage claim. That is not criminal, since I thought it was mine. If I knew that it was not my suitcase and took it anyway, that is theft.
You can also see the importance of criminal intent (called “mens rea”) in terms of whether a killing is 1st degree murder, 2nd degree, manslaughter, or negligent homicide. The same act (killing another) can be any one of several different crimes depending on the defendant's state of mind when he/she committed it--or might not be a crime at all, if the killing happened in the course of legitimate self-defense, since then there was no criminal (wrongful) intent at all.
In the check-cashing context, what makes it criminal is intent. Intent in this context is based on knowledge. Did you know, or at least have reasonable grounds to know, that the check was no good? If you did not, it was not a crime. You would still have to repay any money you received from cashing the check, because you are NOT allowed to keep money from a check unless there was sufficient money in the account from which it was drawn to cover it.
What happens if you "unknowingly"--that is without knowledge that the check is bad--cash it. If you truly did not know, you did not commit a crime. But the authorities (e.g., the police) do not have to take your claims of a lack of knowledge at face value. They look at the facts and circumstances to see if they support your claim.
Or if, instead, the facts and circumstances indicate that you did or should have known that the check was bad. For example:
1) If it was a check given to you by someone else. If the check is large compared to what it is for (e.g., someone supposedly is paying you $500 for something which realistically is valued much less than that), that is evidence that the check is bad and that you should have been aware of that fact. People don't ordinarily pay more than they have to.
2) if the check is from someone that you know has passed bad checks in the past. That is also evidence you should have known that this check was not good.
3) If the check was from someone who told you that they were not sure they'd have money to cover it, or that they were waiting for their next paycheck or a tax refund to have enough money, but you cashed it before they confirmed they had the funds. That would also be reason to conclude that you knew or should have known that the check was bad.
Though there are many other examples, the point is, it's not enough for you to say that you did not know that the check was bad. Rather, the facts and circumstances must support your claim. If they don't, then you could be charged, no matter how loudly you protest that you did not know.