Contradiction with John 1:18
Exodus 20:1 shows God speaking directly, whereas John 1:18 suggests no one has seen God, implying a lack of direct communication.
John 1:18: No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared [him].
Contradiction with Hebrews 1:1-2
Exodus 20:1 describes God speaking directly, but Hebrews 1:1-2 states that God has spoken through prophets and now through His Son, implying different modes of communication.
Hebrews 1:1-2: God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,
Contradiction with 1 Kings 19:12-13
Exodus 20:1 depicts God's direct speech; however, Elijah's experience in 1 Kings shows God speaking not in overt displays but in a "still small voice," indicating varied manifestations of God's voice.
1 Kings 19:12-13: And after the earthquake a fire; [but] the LORD [was] not in the fire: and after the fire a still small voice.