HP-Autonomy ($11.1B, 2011)

HP overpaid 79% above market value for Autonomy, paying $11.1 billion in 2011 for a software firm later alleged to have inflated revenues by $200 million via accounting fraud. The subsequent $8.8 billion write-off — an 80% impairment — sparked decade-long legal battles that cost HP $600 million more in settlement fees. This loss percentage is marginally less severe than Microsoft-Nokia's (#2) total write-off, but the prolonged litigation and reputational damage make it uniquely catastrophic.