Why assume you're "much less intelligent than Voldemort," in addition to the gap in experience? As others have noted, he makes mistakes all the time. (I was surprised by the news that he actually died and was actually trapped for years, but not by him leaving Harry's wand alone given this previous information.) We know that V had vast experience with magic and secret information about the same, which could support your view but could also be a disadvantage when it came to partial transfiguration. Note that the mistake he knew about with the Horcrux network (and would perhaps have updated on) partly consisted of thinking he could overcome a long-established magical limitation without testing it. Maybe the actual complaint should be that Harry's use of partial transfiguration shouldn't have worked without more testing - though here we know that he based it on vast civilizational knowledge which V had only begun to assimilate.