Peterius It seems like the worst move for black is this sequence. Here, if black plays 'a', obviously white will take the right group. If black plays 'b', white will take the left group at 'c', if black plays elsewhere (tenuki), then white plays 'b' and takes the left group. And if either group is taken then the other will die. But obviously is a bad move... its clearly sacrificing a stone for no reason.
is a mistake. Black could settle for seki by playing at .
White can make seki herself.
Now here as long as black doesn't play 'a' before he has surrounded the white invasion, black is okay. If white plays 'b', black takes at 'c'. If white plays 'c', black plays 'b and white can't suicide at 'a' so the white group is still dead. Even if white attempts to play towards the left group, white simply doesn't have the liberties to take it. Please correct me if I am wrong about this, but I don't see a white strategy as long as black pays attention.
Karel? Yeah, you can't even make an approach ko of that or, in fact, do anything really; pity we don't know the context. I'm thinking it was probably used as a ko threat; a high dan wouldn't seriously use that as an attack on the group of an opponent of the same level - that'd be nearly insulting.
JoelR: Black can't respond to 'b' with 'c'; it allows an approach ko. White makes a ko threat, and retakes at . Then if White ignores Black's threat and plays at 'd', she has ko for the whole kill.
unkx80: You guys have gotten the correct solution. I will be putting up the solution page soon. =)