If you have a fixture that uses more than one light bulb, it is usually a good idea to use the same type of bulbs in each socket. If you use LED and incandescent bulbs together, the incandescent bulb puts out heat and can ruin the LED. For the sake of frugality, many people buy LEDs and then replace their old bulbs one by one. This is a bad idea if they are going to be used in the same fixture, and it is best to replace them all at once when switching to a different type of light bulb.
This can cause other problems, too. The chip from the LED can overheat and fail quickly. Also, the LED will receive less power from the circuit, and this can cause it to flicker, buzz, or be dim. These difficulties are less with a CFL, but it is still best to use the same type of bulb in every socket.