The deliberate gap between mating teeth that every real gear needs — and the real positioning-error tradeoff it creates.
Cut two gears to mesh with zero clearance and you haven't built a more precise gear train — you've built one that seizes the first time it warms up. Backlash, the small intentional gap left between mating gear teeth, is one of those design features that looks like sloppiness until you understand why it has to be there, and then looks like a tradeoff the moment you need precise, reversing motion control.