I have seen at least two instances so far: that trooper at the Battle of Christophsis, and Cut Lawquane. Yes, they both happened to have lost their firearm but why do they punch the enemy droid in response? Punching a metal droid simply makes the clone's hand hurt badly enough to distract him long enough for the droid to retaliate.
During the battle at the Rishi Station, Rex demonstrated how to take down a commando droid in melee combat. Are most clones not trained in how to properly engage an enemy droid in melee combat, or are they indeed trained in doing something as ridiculous as punching metal with your own hand?