He is promising his friend that he should be prepared to be the victim of an even more ingenious practical joke. This illustrates an enormous problem in interpreting and applying the scripture. The problem is as old as the oldest scripture. Among people who accept the Bible as God's inspired word, disagreements rarely are based on what is said in scripture. People who accept the Bible as God's word commonly agree on the words.
Most disagreements center in the intent of the words.
Always, the context is powerfully related to the intent. Jesus and the Pharisees agreed on what the law said. They disagreed about God's intent in what the law said. Often, the Pharisees declared the law meant only what the words literally said. Jesus commonly emphasized that God's intent went far deeper that the literal wording of the law.