Oopsie Jumpsie is a fast, competitive multiplayer runner built entirely with GenAI-generated 3D models, textures, and audio inside Horizon Worlds. Players jump, dodge, and race side-by-side to see who gets the furthest, with portrait-mode controls designed for quick mobile play. A procedural fragment system keeps every run fresh, while live rankings and haptic feedback reinforce the social, replay-driven experience. Created by a small MHCP team, Oopsie Jumpsie explores how GenAI can accelerate high-quality worldbuilding and production for fun, social-first experiences.