Third there is a flaw, yes there is always the temptation of mounting the hard drives in that manor but isn't recommended due to the uneven wear on the spindle that can lead to early failure.
Desktop Drives: The drive can be mounted in a standard 3.5"device bay. The drive can be mounted sideways, on end, or even upside down as long as the mounting screws are used properly. Use either the four bottom screws or four of the side mounting screws to support the drive. This will prevent vibration and provide additional electrical grounding.