Just an very old one I rewatched recently for some reason, Tra Telligman was short notice against Igor Vovchanchyn in 2001 and shocked the world back then. Igor was 46-3 with one loss in the GP final over the last six years, with a recent win over Mark Kerr in their rematch, into the fight and Tra was 6-3, on a two fights losing streak, last win was more than 3 years old. Igor was supposed to face Ken Shamrock, he endd up against a short notice replacement that felt like a total mismatch who did it in impressive fashion. That's, as far as I'm concerned, the biggest short notice upset ever. Pride 13.