Contradiction with Matthew 6:1
Jesus warns against doing charitable deeds to be seen by others, whereas in Matthew 15:36, giving thanks in front of a crowd could be seen as a public display.
Matthew 6:1: Take heed that ye do not your alms before men, to be seen of them: otherwise ye have no reward of your Father which is in heaven. [alms: or, righteousness] [of your: or, with your]
Contradiction with John 7:4
Jesus' brothers tell Him to show Himself to the world, contradicting His earlier private act of giving thanks before performing a miracle.
John 7:4: For [there is] no man [that] doeth any thing in secret, and he himself seeketh to be known openly. If thou do these things, shew thyself to the world.