[referring to the past lecture that had no lies] "Do you remember the first lecture - how I said that 'every lecture has a lie?'" ... "Well - that was a lie."

What if THAT was the current lecture's lie? :)

I used to work for a small boutique testing/QA shop. They took this approach with almost all their internal documents. In order to get people to actually read and absorb new documents and/or presentations, they would deliberately salt the content with one falsehood. It was a very effective technique.