It is generally not safe to leave apple pie out overnight. Apple pie contains perishable ingredients like dairy and eggs in the filling, which can promote rapid bacterial growth at room temperature. According to food safety guidelines, apple pie should be refrigerated within two hours of serving to prevent the risk of foodborne illnesses. If left out overnight (more than 8-12 hours), harmful bacteria could multiply to dangerous levels, and reheating the pie will not reliably make it safe to eat. To maintain quality and safety, refrigerate apple pie leftovers promptly and consume them within 3-4 days.

If the apple pie is left out for less than two hours, it is generally considered safe to eat, but beyond that, it is recommended to discard it. The presence of sugar and acidity in fruit pies can slow bacterial growth somewhat, but this is not enough to prevent risk if left out too long. Proper storage by refrigeration helps preserve freshness and prevent contamination.