My opponent had two 2 toughness creatures in play, one tapped and one untapped. I attacked with my 1/1 Dreadhorde Butcher. My opponent blocked with their untapped creature. As my Butcher died, I was hovering my mouse over the creature with one damage on it, ready to click it as the target for my trigger. As soon as the trigger resolved, the creatures switched places, and I clicked on the undamaged creature. My suggestion is not to have creatures switch places for absolutely no reason.