Occasionally when playing Caitlyn, her auto attacks will land before the animation is finished. It usually happens while trying to farm early game, although it's harder to notice later on.
I'd put a step-to-step, but honestly I've only seen it happen at random. It tends to happen early game though, if that helps at all.
Got the same bug during a Caitlyn game I played yesterday. Essentially, what's going on is that some or all of Caitlyn's autoattacks have an instantaneous projectile speed. As soon as the animation starts, the target's health is immediately updated, then the projectile finishes its animation, appearing to hit the target for 0 damage. Like the OP said, it's most notable during early game, when Cait's attack speed is still fairly low.
No idea how to recreate the exact circumstances. Just play Cait and farm for a while, and you might notice.
I feel like this just happened to me, maybe I'm crazy but it also felt like her auto attacks took even longer than usual to go off too. or do the damage it should have done.
