Contradiction with Genesis 1:31
While Genesis 6:12 states that all flesh had corrupted God's way, Genesis 1:31 declares that God saw everything He made as very good, suggesting a state of goodness and uncorrupted creation.
Genesis 1:31: And God saw every thing that he had made, and, behold, [it was] very good. And the evening and the morning were the sixth day. [And the evening...: Heb. And the evening was, and the morning was etc.]
Contradiction with Genesis 8:21
This verse mentions that God will not again curse the ground for man's sake because the imagination of man's heart is evil from his youth, implying a continual presence of sin since youth rather than a new corruption of all flesh.
Genesis 8:21: And the LORD smelled a sweet savour; and the LORD said in his heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart [is] evil from his youth; neither will I again smite any more every thing living, as I have done. [a sweet...: Heb. a savour of rest or, satisfaction] [for the imagination: or, through the imagination]
Contradiction with 1 Kings 8:46
States there is no one who does not sin, which assumes sin is inherent and ongoing, contrasting with Genesis 6:12's indication of a specific time of corruption.
1 Kings 8:46: If they sin against thee, (for [there is] no man that sinneth not,) and thou be angry with them, and deliver them to the enemy, so that they carry them away captives unto the land of the enemy, far or near;
Contradiction with Matthew 19:17
Jesus says there is none good but one, that is, God, implying constant imperfection in man, in contrast with an explicit corruption referenced as beginning in Genesis 6:12.
Matthew 19:17: And he said unto him, Why callest thou me good? [there is] none good but one, [that is], God: but if thou wilt enter into life, keep the commandments.