Shield stab
A shield stab is the event of finding a damageable portion of a character slightly outside of their shield and striking it, therefore bypassing their attempt to shield. This is an extremely useful technique, as it can be used to strike an otherwise defended enemy.
Some characters are easier to shield stab than others. For example, Mr. Game & Watch's shield is small compared to his body size (though this was later changed in Brawl, where his shield does cover all of his body), and Captain Falcon and Pit are too tall for a spherical bubble to cover properly. A character that is short and/or round (such as Kirby or Olimar) can be hard to shield stab for similar reasons.
If a character sees an incoming shield stab attempt, they may be able to shift their shield to cover up the area. This gets harder as their shield weakens and shrinks.
It should be noted that Yoshi, due to his unique shield (encasing himself in an egg), cannot be shield stabbed.