Preheat oven to 350°F. Grease a 9x13-inch pan with nonstick spray. Set aside.
15.25 ounces yellow cake mix, ½ cup vegetable oil, 3 large eggs, 1 cup water
Prepare the cake mix with the water, oil, and egg. Bake per the box directions (about 30 minutes). Allow the cake to cool for 10 minutes.
Take the back of a wooden spoon or a chopstick and poke a hole almost to the bottom of the cake. Repeat this process all over the cake. Set aside.
In a medium bowl, whisk together the pudding mix and milk just until combined. Immediately pour the pudding evenly over the cake.
6.8 ounces instant banana pudding mix, 4 cups milk
Cover the cake with plastic wrap and place it in the refrigerator for at least 1 hour to let the pudding set.
Remove the cake from the refrigerator. Spread the whipped topping over the top of the cake.
8 ounces whipped topping
Place some whole wafers on top of the whipped topping, then crush the rest of the wafers and sprinkle the crumbs over the top. Place the banana slices over the crumbs prior to serving.
1½ cups vanilla wafers, 1 banana