(;
AB[rn][ro][rp][rq][pp][qp][kq][mq][oq][or][ss]
AW[ql][rm][pn][qn][sn][oo][op][pq][pr][qq][rr][qs]
C[Black plays.];
B[os]
C[An obvious 1st move.];
W[ps];B[sr]
C[An obvious 2nd move.];
W[sq];B[sp];W[po]
(;
B[rs]
C[But this nice tesuji is easy to miss from the original position.];
W[sq]
(;
B[rs]
C[Another easy-to-overlook tesuji.];
W[ss]
C[Black has a ko.

Note that this ko is direct for White, who only needs to connect to win the ko.

However, ...];
B[sr];W[hh]
C[... after White makes a ko threat, ...];
B[rs]
C[... and Black ignores the threat in order to capture here, then the ko becomes direct for Black and a 2 step ko for White.]
)
    (;
    B[sr]
    C[Mistake.];
    W[rs];B[sq];W[qo]
    C[White wins the semeai.]
    )
)
    (;
    B[sq]
    C[Mistake.];
    W[qo]
    C[White captures the corner.]
    )
)

