Anakin murdering younglings.
Not to pick a nit, but we never actually see the dastardly deed.
I’m pretty sure we see the hologram of it. I haven’t seen ROTS for over ten years, so I’m not 100% sure, though.
I have seen it fairly recently and they don’t show you, instead they show Anakin/Vader killing the masters and not the younglings. Of course I might be wrong about this, so I would have to recheck the scene in question.