I was explaining the first two paragraphs of the introduction to Immanuel Kant's Critique of Pure Reason to my large Introduction to Philosophy class. It was midway through the spring semester of 1992, give or take a few years. The first paragraph begins with, "There can be no doubt that all our knowledge begins with experience." The second paragraph begins with, "But though all our knowledge begins with experience, it does not follow that it all arises out of experience."
I explained the ideas in the first paragraph—we cannot have knowledge unless we first have sense experience—and gave an example or two. This claim is made by empiricists, often to support their view. But Kant is no empiricist. He thinks that even though we cannot have knowledge without first having sense experience, some of our knowledge is not derived from that experience. So the second paragraph is an important qualification of the first.
After explaining the ideas in the first paragraph, I said, "And now comes the big but."
The class roared with laughter. For a second I was puzzled. Then I realized my faux pas. I paused for a bit, smiled wanly, then proceeded with my explanation of Kant.