Cross-reactive antibodies targeting multiple epitopes have been identified in Sarbecoviruses, but the precise molecular mechanism(s) behind the crossreactivity remain poorly understood. Here, we isolate 3D1, a broadly neutralizing antibody (bnAb) derived from a human combinatorial antibody library targeting the conserved HR1 domain. 3D1 uniquely recognizes a β-turn fold comprising a 6-mer peptide (pepDVVNQN/Q) that forms during a pre-hairpin transition state, occurring exclusively before membrane fusion during viral infection. 3D1 effectively neutralizes a wide range of live SARS-CoV-2 wild-type strains except for Omicron, which evades neutralization due to a detrimental point mutation (Q954H). Notably, this cryptic epitope reveals a signature motif that extends throughout the core region of coronaviruses and is also present in various RNA viruses, including HIV and Marburgvirus. 3D1 functions as a natural or background antibody capable of binding to a diverse array of non-self antigens. 3D1's cross-reactivity underscores the effectiveness of the library approach, which encompasses the entire antibody repertoire.