Contradiction with Mark 4:34
Jesus explained everything to his disciples in private, contrasting the disciples' lack of understanding in Matthew 15:15.
Mark 4:34: But without a parable spake he not unto them: and when they were alone, he expounded all things to his disciples.
Contradiction with John 16:29-30
The disciples claim that Jesus' teachings are clear and understandable, opposing their confusion in Matthew 15:15.
John 16:29-30: His disciples said unto him, Lo, now speakest thou plainly, and speakest no proverb. [proverb: or, parable]
Contradiction with Matthew 13:11
Jesus tells the disciples that it is given unto them to know the mysteries of the kingdom, unlike their apparent lack of understanding in Matthew 15:15.
Matthew 13:11: He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.