sorry, but it's not a pattern error- it's a sewing error- many blocks are put together exactly this way- and many of us have had to deal with this little problem- it take (practice) i guess to get these right.
you need to really line up exactly- and stitch a (scant) 1/4" on each side of that line- if you are the least bit too much you will find them turn out just like yours did- what are you doing with the 1/2 square triangles you are cutting off? are they used later?--one fix is to stitch on the diagonal line - but if you need those smaller pieces later that technique will not work- you will have to find your (sweet spot)- this is one reason i never cut out all of the pieces for a quilt at once- i cut enough for a couple blocks and try it out- in case i need to cut something a tad larger to make something work.
if you have all of your pieces cut out-and your diagonals all drawn- adjust your seam line on each side of the diagonal to be a bit smaller- it will help- and press toward the corner-not toward the pieced block.
i hope that helps....it just takes some practice---slow down and check each block to make sure it's where you need it before you cut it- adjust until you have it right- everyone sews a little differently- and it takes finding the measurement that works for you.