Originally Posted by wolfkitty
I sometimes get in a hurry, and get a bit careless on my 1/4 seams, and I think that throws me off. Maybe that's part of it.
That's definitely part of it. If your seams are not all exactly the same, corners will not match. Consistency is extremely important. Eleanor Burns says you can fudge a little and ease or stretch the piece to fit. The other suggestions of locking seams and using a pin are very good. I use a pin when I am sewing triangles. I learned this from Fons and Porter.