Punishment that's too harsh becomes overcompensation and ends up hurting morale.
Tips
history
From the 《后汉书·仲长统传》 (Book of the Later Han): in trying to set things right after a chaotic age, reformers fall into 矫枉过正 — bending the bent stick so far the other way that it's bent again.
memory
Picture straightening a bent (枉) stick: push too hard and you've now bent it the other direction (过正 = past straight). The mistake isn't the correction, it's the excess.