Matt. 13:31-2 Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
It is argued that Jesus was wrong here because there are smaller seeds in existence, like the orchid.
But the Greek word here is mikros, and it does not merely have size in view. For example, it is also used in Luke 9:48 -
And said unto them, Whosoever shall receive this child in my name receiveth me: and whosoever shall receive me receiveth him that sent me: for he that is least among you all, the same shall be great.
Obviously, Jesus did not mean here that he who was smallest in size would be greatest. So we suggest that the mustard seed description is not an evaluation of size at all (or exclusively), but an evaluation of worth - which is a matter of personal judgment that is quite acceptable and cannot be charged as erroneous.
Two other objections may crop out of this one. First, some versions refer to the mustard as a "shrub" but the word here is lachanon, meaning a vegetable or an herb. (Rom. 14:2 "For one believeth that he may eat all things: another, who is weak, eateth herbs.")
A second objection is that mustard becomes a tree. Of course, the scientific classification schemes of modern botany were not yet invented -- classification was by appearance and function, not by biology. In that regard, the mustard plant of Palestine (Sinapis nigra) could grow to a height of several (2-6) feet and be considered a tree -- if not by the scheme of modern science (Is a bonsai a "tree"?), then assuredly by the descriptive classification scheme of the ancients.