A miss shift can happen if you re-engage the clutch prior to the gears/synchros getting into position while shifting. You are essentially between gears. For instance, you are shifting down from third into second, yet release the clutch pedal too early during the shift. Your shifter would be in the "neutral" stance (between gears). You then reapply power through the transmission, but it doesn't transmit because there's nothing connected between the input and output shafts. At this point, if you continue to move the shifter down through to second gear after the clutch pedal is released and starting to apply power, this is where you get the classic "grinding gears" ... in most cases with today's transmissions, you're actually grinding the dogs on the synchros and not the gears, but the noise is awful either way. This can happen going up or down through the gears. It will most often happen when doing a shifting crossover from 2nd to 3rd, or 4th to 5th and visa-versa, not when shifting from 1st to 2nd or 3rd to 4th. This is because it usually takes less time to shift between these gears with them being in a straight line.