>A mistake of law by a police officer sometimes can trigger the exception. If an officer takes steps based on the existing interpretation of the law, but a court later rules that the law should be interpreted differently, they may be found to have acted in good faith.
Your mainstream explanation isn't correct. This only applies when a court replaces an existing precedent - a relatively rare occurrence. Police shouldn't be expected to anticipate the court's change of heart ahead of time.
Most people can’t afford an attorney and end up taking a plea.