Posting under the name Bryan Klouda, a visitor to my blog A Proof that the Christian God Does Not Exist recently left the following comment:
Haven't read the comments or the entire post, but got to the square circles part.
Out side of the context in which the argument is being made, square circles do exist. A shape with 250 sides can be considered a circle, just like a shape with 4 sides can be said to be a circle. Since a circle is defined as something like every point on the circle is equidistant from its center, it can be said that no circle even exists because you can never have a perfect circle, and if the shape doesn't fit the criteria of the definition, it follows that it must be something else.