Contradiction with Matthew 8:23
Matthew 15:39 describes Jesus sending away the multitude and departing by ship, while here Jesus enters a ship with His disciples without mentioning a crowd.
Matthew 8:23: And when he was entered into a ship, his disciples followed him.
Contradiction with Matthew 14:22
Matthew 15:39 depicts the disciples following Jesus after He sent the multitude away, but here, He constrains His disciples to get into a ship and go before Him while He sends the multitude away.
Matthew 14:22: And straightway Jesus constrained his disciples to get into a ship, and to go before him unto the other side, while he sent the multitudes away.
Contradiction with Mark 6:45
In Matthew 15:39, Jesus departs with His disciples following, whereas this verse highlights Jesus instructing His disciples to go to the other side ahead of Him without the crowd.
Mark 6:45: And straightway he constrained his disciples to get into the ship, and to go to the other side before unto Bethsaida, while he sent away the people. [unto...: or, over against Bethsaida]
Contradiction with John 6:16-17
Matthew 15:39 involves Jesus departing with His disciples after sending away the multitude, contrasting with these verses where the disciples go to the sea in a ship without mention of Jesus or multitude being sent away first.
John 6:16-17: And when even was [now] come, his disciples went down unto the sea,