Contradiction with John 18:20
Jesus states that He always taught in the synagogue and temple, not specifically in a "plain" as described in Luke 6:17.
John 18:20: Jesus answered him, I spake openly to the world; I ever taught in the synagogue, and in the temple, whither the Jews always resort; and in secret have I said nothing.
Contradiction with Matthew 5:1
Describes Jesus going up a mountain to teach, which contrasts with Luke 6:17 where He stands on a plain.
Matthew 5:1: And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him:
Contradiction with Acts 1:8
Instructs the disciples to be witnesses in Jerusalem and beyond, implying a broader and varied teaching approach than just in a plain.
Acts 1:8: But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth. [power...: or, the power of the Holy Ghost coming upon you]
Contradiction with Matthew 13:2
Indicates Jesus taught from a boat on the sea, contrasting with teaching on a plain.
Matthew 13:2: And great multitudes were gathered together unto him, so that he went into a ship, and sat; and the whole multitude stood on the shore.
Contradiction with Mark 4:1
Also describes Jesus teaching by the sea, differing from the plain setting in Luke 6:17.
Mark 4:1: And he began again to teach by the sea side: and there was gathered unto him a great multitude, so that he entered into a ship, and sat in the sea; and the whole multitude was by the sea on the land.
Contradiction with Mark 6:6
Mentions teaching in various villages, not specifically in a plain.
Mark 6:6: And he marvelled because of their unbelief. And he went round about the villages, teaching.