This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

The flood lasted 40 days.

Genesis 7:17 View context

And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth.

The flood lasted 150 days.

Genesis 7:24 View context

And the waters prevailed upon the earth an hundred and fifty days.

Genesis 8:3 View context

And the waters returned from off the earth continually: and after the end of the hundred and fifty days the waters were abated.