An Invalid Obstruction occurs whenever a strike is blocked by something that is not allowed block or otherwise affect gameplay in such a manner. This includes but is not limited to: hair (ponytails, beards, etc), loose garb, bulky or protruding armor & accessories, unwielded equipment, non-equipment items such as water bottles, and so on.
A strike that is prevented from making contact due to an Invalid Obstruction counts as a valid strike against whatever would have been hit if the obstruction was not there, as well as a strike against the obstructing object itself if it is able to be affected, such as a sheathed dagger being struck by a Weapon Destroying effect.
For ambiguous incidents where identifying what 'would have been' hit is not so easy, such as two projectiles colliding mid-air, the players involved should do their best to resolve the situation quickly and in good faith. In the example presented here, this is usually done by simply disregarding both projectiles; however, this is not a rule and each case should still be considered independently.