"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.
The disciples are so much like us. We argue and fret over the most trivial things while Christ is trying to get us to pat attention to things of eternal significance. We argue over the color of carpet or whether it's appropriate to bless animals, while people are hungry and hurting in the community around us and around the world.
The disciples were fretting about missing a meal while Jesus was concerned about teachings that freed others from the oppressive rules of the religious leaders. We fret over budgets while Jesus tells us to trust him to provide all we need. We argue about whether to sprinkle or dunk when baptizing when Jesus would have us preach his word to those who are lost. We debate the theology of infant baptism while children suffer abuse in our own communities. We complain if something is changed in the order of worship while others are dying for worshiping Christ. We criticize how our youth dress for church instead of being thankful that they are there to worship God.
And what does Jesus tell us? Maybe we need to beware the yeast we are spreading, the bad yeast that poisons our churches and harms the cause of Christ in our world.