A case that surprised me during a post-game review, where either a passive or an aggressive move can kill white.
(;SZ[19]AB[pb][qd][pe][od][ne][mf][lf][ke][jd][jc][kb][ib][re][kf]AW[pc][oc][nd][md][mc][me][le][lc][kc][kd]AE[jg][jf][if][ie][id][ic][je][qe]CR[pc]C[Where should black move so white cannot make a second eye?](;B[qc];W[ob](;B[qb];W[mb]C[White makes a second eye.])(;B[qa];W[lb];B[nb];W[oa]C[White makes a second eye.])(;B[pa];W[mb]C[White makes a second eye.])(;B[mb];W[qb](;B[nb];W[lb];B[la];W[ma];B[na];W[nc]C[White makes a second eye.])(;B[lb];W[rb]))(;B[lb];W[mb]C[White makes a second eye.])(;B[nb];W[qb];B[lb];W[mb];B[ma];W[nc];B[na];W[oa]C[White makes a second eye.])(;B[ma];W[qb]C[White makes a second eye.])(;B[na];W[nb];B[mb];W[qb]C[White makes a second eye.])(;B[la];W[mb]C[White makes a second eye.]))(;B[nb];W[ob](;B[qb];W[oa](;B[lb];W[mb]C[White makes a second eye.])(;B[la];W[mb]C[White makes a second eye.])(;B[ma];W[la];B[lb];W[mb]C[White makes a second eye.])(;B[mb];W[lb];B[la];W[ma]C[White makes a second eye.]))(;B[mb];W[lb];B[la];W[ma];B[na];W[oa]C[White makes a second eye, because black can't save both the O18 and the Q18 groups.]))(;B[ma];W[ob];B[qb];W[la](;B[oa];W[mb];B[nb];W[nc]C[White makes a second eye.])(;B[lb];W[mb](;B[nb];W[oa]C[White makes a second eye.])(;B[na];W[nb]C[White makes a second eye.]))(;B[na];W[nb](;B[lb];W[mb]C[White makes a second eye.])(;B[oa];W[mb]C[White makes a second eye.])))(;B[rb];W[qb](;B[qc];W[ob];B[qa];W[mb]C[White makes a second eye.])(;B[qa];W[qc];B[rc];W[ob];B[pa];W[mb]C[White makes a second eye.])(;B[ob];W[qc](;B[nb];W[mb](;B[qa];W[rc];B[nc];W[pa];B[oa];W[ma]C[White's group is alive.])(;B[rc];W[pa]C[White's group is alive]))(;B[qa];W[nb];B[rc];W[oa]C[White's group is alive.])))(;B[lb];W[ob](;B[qb];W[mb]C[White makes a second eye.])(;B[mb];W[qb]C[White makes a second eye.]))(;B[ob];W[nb](;B[qc];W[qb];B[rb];W[lb];B[ma];W[ka];B[jb];W[na];B[la]C[RIGHT])(;B[qb];W[qc];B[rc];W[rb](;B[sb];W[ra];B[qa];W[oa];B[sa];W[lb];B[ma]C[RIGHT])(;B[ra];W[lb];B[ma]C[RIGHT]))(;B[rb];W[qb];B[qc];W[oa];B[qa];W[lb];B[ma]C[RIGHT]))(;B[qb];W[ob](;B[mb];W[qc];B[rc];W[lb];B[la];W[ma];B[na];W[nb];B[ma]C[RIGHT])(;B[lb];W[mb]C[White makes a second eye.])(;B[na];W[mb](;B[oa];W[nb]C[White makes a second eye.])(;B[ma];W[nb]C[White makes a second eye.])(;B[nb];W[oa]C[White makes a second eye.]))(;B[ma];W[la](;B[oa];W[mb](;B[na];W[nb]C[White makes a second eye.])(;B[nb];W[nc]C[White makes a second eye.]))(;B[lb];W[mb](;B[nb];W[na]C[White makes a second eye.])(;B[na];W[nb]C[White makes a second eye.])(;B[ka];W[nb]C[White makes a second eye.])(;B[oa];W[na]C[White makes a second eye.]))(;B[na];W[mb](;B[nb];W[nc]C[White makes a second eye.])(;B[oa];W[nb]C[White makes a second eye.]))))(;B[na];W[ob];B[qb];W[mb](;B[oa];W[nb]C[White makes a second eye.])(;B[ma];W[nb]C[White makes a second eye.])(;B[nb];W[oa]C[White makes a second eye.]))(;B[mb];W[lb](;B[la];W[nb];B[ma];W[qb]C[White makes a second eye, because black's Q18 stone cannot be saved.])(;B[nb];W[ob];B[la];W[ma];B[na];W[oa]C[White is alive, because black cannot save both the O18 and the Q18 groups.])))