Our Lord gave us some cautions so that we would beware. In Matthew 7:15 it states, "Beware of false prophets, who come to you in the clothing of sheep, but inwardly they are ravening wolves." There are two ways a prophet can be false: First, having a mandate to teach, he teaches his own or some other false doctrine rather than the teachings of the one who sent him. The second way a prophet might be false is if he does not have a mandate to teach. And our Lord uses a modifier to tell us which false prophets: Those "in the clothing of sheep". Our Lord does not warn us of those in shepherd's clothing, but in sheep's clothing. This would imply the second kind of false prophet, the kind without a mandate to teach, and thus susceptible to error.
Our Lord also warns about the leaven of the Pharisees. They understood that he said not that they should beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees. The Pharisees fancied themselves the religious rock stars of the day, supposedly the holiest of the holy. And they made sure everyone else knew it. But the Lord called them out on their hypocrisy.