In a video posted on Reddit, an unlucky Overwatch player got destroyed by a confused Sigma's mistaken rock throw. The Sigma was being tossed around by a Wrecking Ball and clearly didn't plan to hit the Lucio when he threw the rock.
They probably didn't even mean to throw the rock in general.
The video shows a Lucio sneaking around the side of the map by wallriding, but as they turn the corner, they are hit by a Sigma rock and fall off the map. It happened far too quickly for the Sigma to have known the Lucio was there or aim at him which makes the clip pretty incredible.
However, it is hard to ignore just how unlucky the Lucio is or if the Sigma knew they would come that way. Even if the Sigma predicted the Lucio would come that way since this is at least the second round of the game, the timing is impeccable. None of his teammates could have seen or heard the Lucio when he threw the rock.