Obrady,
It is covered here and does require a judgement call on the part of the official scorer.
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ERRORS
10.13
An error shall be charged for each misplay (fumble, muff or wild throw) which prolongs the time at bat of a batter or which prolongs the life of a runner, or which permits a runner to advance one or more bases.
NOTE (1) Slow handling of the ball which does not involve mechanical misplay shall not be construed as an error.
NOTE (2) It is not necessary that the fielder touch the ball to be charged with an error. If a ground ball goes through a fielder’s legs or a pop fly falls untouched and in the scorer’s judgment the fielder could have handled the ball with ordinary effort, an error shall be charged.
NOTE (3) Mental mistakes or misjudgments are not to be scored as errors unless specifically covered in the rules.
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If we are talking about a lazy fly ball that the outfielder should have handled with ease, but mis-judged it and ended up diving at it, it should be ruled an error.
It all comes down to this phase...
"in the scorer’s judgment the fielder could have handled the ball with ordinary effort"