Originally Posted by gale
I must be really dumb today because I can't figure out what the mistake was.
It appears as if the sewing was done down through the diagonal of the block. When cut on each side, what was left was unsewn triangles of the wrong size.
What was meant to do was draw a line down the center (rather than sew), then sew 1/4 " on each side of the line, then cut the line and you would end up with two half square triangles. In order for this to work, you need to have cut the squares large enough for you to end up with half square triangles the correct size.