They had forgotten to bring bread, and they had only one loaf with them in the boat. He enjoined them, “Watch out, guard against the leaven of the Pharisees and the leaven of Herod.” They concluded among themselves that it was because they had no bread. When he became aware of this he said to them, “Why do you conclude that it is because you have no bread? Do you not yet understand or comprehend? Are your hearts hardened? Do you have eyes and not see, ears and not hear? And do you not remember, when I broke the five loaves for the five thousand, how many wicker baskets full of fragments you picked up?” They answered him, “Twelve.” “When I broke the seven loaves for the four thousand, how many full baskets of fragments did you pick up?” They answered (him), “Seven.” He said to them, “Do you still not understand?” (Mk 8:14-21)
The Pharisees were asking for a sign. But the disciples were slow in understanding. All what Jesus had done so far were not sufficient for the Pharisees. Still they wanted more signs or they wanted signs according to their liking. They were not willing to accept all want happened as signs.
Now the disciples were not able to understand what was happening around them. Jesus warned them about the leaven of the Pharisees. The Jews used leaven to make ordinary bread. But at Passover time they were forbidden to use leaven to remind them of the hurry with which they were to leave slavery. So warning of the leaven of Pharisees was, in a sense, a reminder of the slavery !!!.
It was also a reminder of the message that there was a growing hostility between the Pharisees and Jesus. Jesus was soon to move towards Jerusalem. He had a mission to complete and in this mission he was in opposition with Pharisees and Herod. The disciples were made to realize that the Pharisees and Herod were in the second place to the kingdom that Jesus was preaching.