Not letting the loaf cool before slicing
You've been patiently waiting for your banana bread to bake for about an hour. It's natural to want to cut into it right away so you can enjoy your treat, but you'll have to wait a little longer. Before cutting into banana bread, allow it to cool completely. Allow your banana bread to cool in the pan for 10 minutes before removing it from the oven. The caramelization of the bananas during baking will make removing the loaf from the oven too difficult.
When you flip the pan over, it is also likely to fall apart. That initial cooling period allows the steam to condense and the loaf to solidify sufficiently to remain intact. Invert the pan onto a plate or small cutting board after 10 minutes. Lift the pan gently to release the bread before turning it over onto a wire rack to cool completely. This allows the loaf's structure to fully coagulate before slicing. If you like your banana bread warm, you can always microwave it for a few seconds.