Contradiction with Genesis 6:4
This verse describes the existence of giants (Nephilim) before the flood, suggesting they have a different origin than the "Zamzummims" described in Deuteronomy 2:20.
Genesis 6:4: There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare [children] to them, the same [became] mighty men which [were] of old, men of renown.
Contradiction with Numbers 13:33
Here, it mentions the presence of giants (the sons of Anak) in the land of Canaan, contrasting with the assertion that the Zamzummims were the giants inhabiting the land before the Ammonites.
Numbers 13:33: And there we saw the giants, the sons of Anak, [which come] of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.