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Batter hits a strong fly ball to the right fielder who runs to catch the ball. The fielder touches the ball with the tip of his glove but could not make the catch.
Would that situation be marked as an error or a hit?

Goes back to ordinary effort. Touching the ball with the glove is not an automatic error. Was the OF running at full speed to try and make the catch? Was he fully extended, or should it have been a easy catch and he misjudged the arc of the ball? Also, what is the age and expected skill level? Judgment call for scorekeeper based on all these things.

Follow up question. Assume a HS game, would you give an error in the above situation if it was a bad left fielder that a normal HS outfielder would catch?

Yes. Ordinary effort should be based on the level of play, not the individual player's ability. So if most HS players in that position would catch the ball without extraordinary effort, then the play would be an error.