Contradiction with Matthew 21:21
Matthew 21:21 suggests that faith itself can move mountains, implying no special method or fasting is needed, contradicting the implication in Matthew 17:19 that the disciples needed more than just faith, such as prayer and fasting.
Matthew 21:21: Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this [which is done] to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.
Contradiction with Luke 17:6
Luke 17:6 implies that faith the size of a mustard seed can achieve great things, contrasting Matthew 17:19 where more than faith seems required for success.
Luke 17:6: And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you.