[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.