Skeptics claim that Mark 4:31 is an error, that the mustard seed is not the smallest seed on earth. But keep in mind that Jesus is not leading a botany class—He is explaining the kingdom of God using an analogy that would be understood by His audience. He is saying that of the seeds sown by the Jews in Israel, it is the smallest. His point is to show the great expanse of the kingdom of God from the smallest of beginnings.
Mark 4:31 It is like a mustard seed which, when it is sown on the ground, is smaller than all the seeds on earth;