This is the real game.
(W250) is normally a sensible move. It is based on a popular end-game strategy based on the concept of miai points - because 
and 
are of exactly the same value (2 points in gote), white can play first (1 point in reverse sente) - even if is significantly smaller white won't lost anything. However, as can be seen, created many ko threats there and eventually white lost the game because he did not have enough ko threats. That's probably why in many commentaries was considered to be the losing move.
and 
are the only remaining endgame moves, one can see black has one more point than in the real game. But as seen in real game, the special area-scoring-technique is worth more than 1 point.
- black cannot save that stone. If 
instead, there are still in total 4 ko threats.
assuming that black can always get the exchange of 'a' and 
is a forcing move. Then 
seems to be a good way to create additional ko threats. If white ignores that, black continues with 
. Because white would have no eye left so it needs to fill many points to capture black assuming it can capture black at all.
is the largest move now. It destroys one point. Then should white get 'a' it can further destroy one point at 'b'. So 
is one way of playing. 
with one point. If white wants that point, 
becomes sente so all the ko threats there are destroyed.