Lance Armstrong for Nike "Livestrong"

Nike’s Livestrong campaign became the costliest endorsement collapse in sports history. The partnership, once hailed as a charity model, crumbled faster than any other on this list, with 90% of Livestrong sales dropping within months. Armstrong’s betrayal cost the brand not just money but consumer trust, a data point that underscores how deeply a single athlete’s deception can wound a global icon.