The failure of tailings storage facilities (TSFs) can cause fatalities, environmental damage, and major financial liability. For industry, a central challenge is to identify credible instability mechanisms before collapse. Recent catastrophic failures of TSFs show that stability assessments must distinguish instability-onset strength from the lower residual or post-liquefaction strength governing runout. Conservative “design-it-out” practice often adopts residual strength for brittle or contractive tailings, however, this can obscure the mechanism and strength condition at which instability initiates. This study presents 2-D limit-equilibrium back-analyses of three well-documented TSF failures: Fundão (2015), Cadia (2018), and Feijão Dam I (2019). Pre-failure geometries were reconstructed from published expert reports and analysed in Slide2 using the Morgenstern-Price method. The back-calculated normalized strength ratio is interpreted as an instability-onset strength ratio, 𝑅IN, defined as the equivalent 𝑠𝑢/𝜎𝑣 ′ required to bring a specified failure mechanism to limiting equilibrium. For Fundão, Cadia, and most Feijão analyses, limiting equilibrium was approached at an equivalent 𝑅IN of approximately 0.20 to 0.26. Feijão Section 3-3 required a higher equivalent value of about 0.35, reflecting the influence of section geometry and material zoning. The results support using 𝑅IN as a mechanism-specific triggering avoidance criterion within credible failure mode assessment, while recognizing that deformation analysis remains necessary where progressive failure, strain weakening, or deformationinduced liquefaction is plausible. This separation helps industry move beyond a single conservative strength assumption and clearly assess whether a credible TSF failure mechanism can initiate.