I feel your pain. I mostly do scrap quilts nowadays so can add small amounts to my stash with no problem. I think maybe if you know you are going to cut it right away then buy less fabric and go back to the store if you need more. The problem of course is if there isn't much fabric on the bolt at the store or if you can't get right to the project.
I just recently screwed up. I made 1100 HST using triangulations software making 12 of the same fabrics at a time. Somehow I came up with 2 1/4" HSTs instead of 2 1/2" My fault I am sure, but irksome as I had thought I would make a lot so that I had a large variety in stash ready to add to projects. They look cute and will work, but I should have checked the size with the first batch! kicking myself.