When they went across the lake, the disciples forgot to take bread. "Be careful," Jesus said to them. "Be on your guard against the yeast of the Pharisees and Sadducees."
They discussed this among themselves and said, "It is because we didn't bring any bread."
The disciples had forgotten to bring bread, except for one loaf they had with them in the boat. "Be careful," Jesus warned them. "Watch out for the yeast of the Pharisees and that of Herod."
They discussed this with one another and said, "It is because we have no bread."
Were the disciples to beware of the Sadducees or Herod?
The disciples were to beware the teachings/attitudes of the Pharisees, Sadducees and Herod. (The Pharisees and Herodians conspired together against Jesus - see Mk 3:6.) Jesus' warning may have been part of a longer statement in which he warned the disciples about all three, but the teaching was condensed to one sentence when it was written down.
Did the disciples have one loaf of bread, or no bread?
The disciples had one loaf of bread, but compared to the amount of bread they needed (i.e. food for at least thirteen people for the time they were on the boat) one loaf was virtually nothing. This appears to have been the disciples' attitude, since they said in Mk 8:16, "We have no bread." Also, it could be that the disciples forgot to bring bread with them onto the boat, but happened to have one loaf already on the boat.