For a rail fence, I would just trim. It does mean something was off in your cutting, sewing or ironing.
If you have done everything correctly, every rail fence block will be square. My bet is that they are rectangular, and that's why you have this problem when you sew two together. Your best bet is to square your blocks before sewing them together.
Actually, if I look at the two blocks in the picture, it appears that your seam allowance is a little too small. That would make your blocks slightly rectangular rather than square. The block on the left has the length you cut; the block on the right has horizontal seams which do not take up enough of the cloth to match the original cut length of the left block.
To get even more particular, it looks as if the beginning/ending of your piecing seam is not exact. It looks as if just the edge of the right block is too big, which means your seam allowance is not uniform. Turn the block over and check the width of the seam allowance at both ends and the middle.