I cast Redcap Melee targeting a nonred creature and was not prompted to sacrifice a land as required. I was playing Pioneer against a mono black player and I targeted their Gifted Aetherborn with my Redcap Melee on my turn 3.

I had three mountains, a Monastery Swiftspear, a Soul-Scar Mage with a +1/+1 counter on it, and a transformed Kumano Faces Kakkazan, and I started on the play. My opponent had played 2 Snow-Covered Swamps and the Aetherborn at that point.

Honestly they were dead on my next turn even if I didnt have the extra land, but I felt so bad about a bug in my favor regardless that I just conceded. I didnt take any screenshots because it didnt cross my mind, I was kind of flabbergasted