sometimes cho'gath's scream doesnt mute/silence. the damage is applied, so it HIT, but they dont get muted/silenced. this tends to happen the most if they are near the side of the cone or at the far end of it. occasionally at point blank as well. seems random to me apart from being more often on those edges.
perhaps because i scream and then right click to move RIGHT after? because i do that alot.
also one time i screamed karthus' dead body (or maybe RIGHT before he died, like portion of a second before) and his ult went off, but did NOT do ANY damage.