I've been letting student use note cards for their exams for the last twenty years. I 'm definitely in the "Here's enough rope to hang yourself" camp. I pass out these 8 inch by 5 inch note cards about 1 week before each exam. The student can put anything they want on their cards. Although, I do tell them that diagraming the entrails of a cat might be counter-productive in a Physics exam. Here are some examples of what I get. On the left is a Environmental Science note card and on the right is a Physics note card. These are both pretty standard. After the exam I require students to turn in their cards, just to see how much is getting through.

However, it never fails I catch one or two student in the hallway just before the exam scribbling frantically on a blank not card. As if the last 10 minutes before class is going to allow you to write the Rosetta stone of all note cards. Of Course, some just brain fart all over the page and come up with something like: