You now have a design opportunity depending upon how my squares you pre-cut the corners off of. You could:
1. square up the incorrectly sewn snowball squares and use them in another project;
2. square up the larger squares that you have already cut the corners off of and (squares will now be smaller) and start over makeing smaller snowball blocks. This will result in a smaller quilt or if you have enough fabrick to coordinate just add more blocks to make the size you intended.
Make it a learning experience and move on!