I thought for sure this plant was a division of my Euphorbia polychroma, but don't remember seeing those showy yellow bracts (you see just the aftermath of bloom here), and the leaves don't look right either, compared to all the images on the ATP species page. Where did I mess up? What have I got on my hands?

Not a Euphorbia expert but the nearest I grew before I moved to this cold zone was Euphorbia robbiae.