Matthew 16:5-11 NLT
 Later, after they crossed to the other side of the lake, the disciples discovered they had forgotten to bring any bread.  “Watch out!” Jesus warned them. “Beware of the yeast of the Pharisees and Sadducees.”  At this they began to argue with each other because they hadn’t brought any bread.  Jesus knew what they were saying, so he said, “You have so little faith! Why are you arguing with each other about having no bread?  Don’t you understand even yet? Don’t you remember the 5,000 I fed with five loaves, and the baskets of leftovers you picked up?  Or the 4,000 I fed with seven loaves, and the large baskets of leftovers you picked up?  Why can’t you understand that I’m not talking about bread? So again I say, ‘Beware of the yeast of the Pharisees and Sadducees.’”
Reflection . . .
So, what is Jesus talking about here in these verses if he’s not talking about bread?
Twice Jesus mentions the yeast of the Pharisees and Sadducees. Bread normally contains some kind of yeast or leavening ingredient for raising and baking purposes, but here Jesus is identifying the leavening that acts to puff up bread as a characteristic of the Pharisees and Sadducees. They were making themselves look better than they were, filled with air and puffed up. They were arrogant and hypocritical, with a holier than thou attitude and with totally works oriented perspective.
While Jesus had done the great miracles of feeding so many people, His disciples weren’t understanding His warnings against the religious leaders and teachers, and not to be like them.
It’s easy to take the position of knowing it all and neglect realizing our deep need for Jesus every moment of every day.