Certainly you would never intentionally leave a baby, child or pet locked in the car when the weather is hot, or anytime.
Within 10 minutes, the temperature in a car can cause heat stroke.
But you could be very busy or preoccupied. A sleeping child in the back seat could be forgotten. The CDC recommends placing a stuffed animal on the front seat to help you remember. Always check to see that everyone is out of the car before leaving.
Leave pets at home. Move your pet into the air conditioned house. Be sure there is plenty of water available.