Contradiction with John 6:12
While Mark 6:42 describes all eating and being filled, John 6:12 emphasizes the collection of leftovers, suggesting more than enough was provided beyond just being filled.
John 6:12: When they were filled, he said unto his disciples, Gather up the fragments that remain, that nothing be lost.
Contradiction with Proverbs 13:25
Proverbs states that the righteous eat to the satisfying of their soul, which could imply that the wicked do not, potentially contradicting the notion in Mark 6:42 where it seems all were satisfied regardless of righteousness.
Proverbs 13:25: The righteous eateth to the satisfying of his soul: but the belly of the wicked shall want.
Contradiction with Luke 6:21
Luke mentions those who hunger shall be filled, which implies a future fulfillment rather than the immediate satisfaction present in Mark 6:42.
Luke 6:21: Blessed [are ye] that hunger now: for ye shall be filled. Blessed [are ye] that weep now: for ye shall laugh.