<p>In a podcast from November 2016 , Barry Gordon himself explicitly states (at 6:14) he only guest-starred on the Hanna-Barbera cartoon a few times and that he was <i>not</i> the voice (or in this case, the <i>secondary</i> voice) of Brainy Smurf.
</p><p>This puts a long-lived rumor to rest, though it brings up the question of how it was started in the first place.
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(Edited by CloudBubble)