It's not an optical illusion, and it's not a sign your vision is faltering. The never-ending tumbling of slices from a loaf of bread atop Aunt Millie's bakery on Pearl Street has stopped. But not for long. Melissa Dunning, a spokeswoman for Aunt Millie's, said this week that the sign stopped turning when a slice of bread was torn loose in a windstorm. A contractor is due to repair the sign Thursday, she said.