Why did Jesus warn the disciples against the leaven of the Pharisees and Sadducees when they forgot to bring any bread?

Matthew 16:5 merely provides some preliminary information that prepares the reader for the episode that follows. The actual event begins in Matthew 16:6, where Jesus issues both a command and a warning to his disciples: Watch and beware of the leaven of the Pharisees and Sadducees.
The warning is introduced with a positive imperative: Watch. This instruction in Matthew 16:6 is closely linked to the incident that has just been concluded where he had been put to the test by the Pharisees and Sadducees (Matthew 16:1–4). They had demanded a sign from heaven, and Jesus had rebuked them. They were able to read the signs of the sky and draw conclusions from them, yet they refused to interpret the far greater signs already evident in his works.
They should have observed what Jesus was doing and discerned its meaning. Instead, out of hostility and unbelief, they deliberately blinded themselves to the truth. Now, as Jesus sails away across the quiet sea, he turns to his disciples with a reflective admonition: Ensure that you truly see—do not turn a blind eye. (see Matthew 13:16–17 for the blessing pronounced on those who see and recognize God’s salvation in Jesus).¹