One defense that clients frequently bring up in Criminal Defense cases is Ignorance or mistake as to law or fact. In terms of mistake of law, it is an almost universal rule in the law that ignorance of the law is not and cannot be a defense. Specifically, it is not a defense to speeding on a highway that you did not know that speeding was a crime in the state of Pennsylvania if the roadway has speed limit signs properly posted along the highway.learn about Murder charges Additionally, it is also not a defense in regards to ignorance of the law if an accused believes that it is legal to possess marijuana in the commonwealth of Pennsylvania when it is not. However, under Pennsylvania law, mistake as to a fact can be a defense. Specifically, it can be a defense in a speeding case if there are no signs on the highway explaining what the speed limit is and the driver mistakenly believes that the speed limit is 75 miles per hour when it is really 65 miles per hour.