Does anyone know if iq tests produce more false positives or false negatives?

Is it possible to score high but not really be all that gifted? Ex. High achiever, practices a lot of standardized tests, sample questions, gets tutoring, etc.

Or more probable that a child is highly gifted but scores poorly on iq test due to poor test taking. Ex. Anxiety, LD masking giftedness, cultural differences, young child not mature enough, etc.