When I took Calc I in school, I was hanging out with another student from Hungary who had done the course already in the old country but somehow needed the cred

When I took Calc I in school, I was hanging out with another student from Hungary who had done the course already in the old country but somehow needed the credit here (in Canada), so he decided to read the "additional reading" that the professor recommended. He discovered that one of the books suggested to instructors to make errors on purpose when demonstrating a calculation on the blackboard. The idea is that once you reach the part where the equation doesn't resolve, you have to go line by line and examine everything carefully to see where the error is, and that's how as a student you better remember the calculus strategy used. The professor who I thought was a distracted older lady was actually a wily teacher :)