This Bible contradiction is from the Skeptic's Annotated Bible.
On a mountain top
Matthew 5:1-3 View context
And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him:
And he opened his mouth, and taught them, saying,
Blessed [are] the poor in spirit: for their's is the kingdom of heaven.
On a plain
Luke 6:17 View context
And he came down with them, and stood in the plain, and the company of his disciples, and a great multitude of people out of all Judaea and Jerusalem, and from the sea coast of Tyre and Sidon, which came to hear him, and to be healed of their diseases;