OK, make sure you have slow motion.

Go to the bit where Snow White is making the pie for Grumpy.

OK, slo-mo the bit where the birds start jumping on the pie. Keep watching, then the birds will slooooly go to the middle of the pie. Watch their feet as they do the final scratches. Their feet go right across where the scratches should be, but there are no scratches. Then, suddenly, out of nowhere, the scratches appear!!!