The following occured in one of my games:

Click Here To Show Diagram Code

[go]$$Bc
$$ ------------------
$$ | . . . . . . . . .
$$ | . . 5 . a . . . .
$$ | . . 4 1 . e b . .
$$ | . . 6 , . . . . .
$$ | . . . 2 . . . . .
$$ | . . . . c . . . .
$$ | . . 3 . d . . . .
$$ | . . . f . . . . .
$$ | . . . . . . . . .[/go]

a and b are two possible continueations for black. Since after b white could play d because the ladder favors him, black seemed to not want to play b. But if he would have played a, white could play c and look forward to either pressure the top with e or the left with f.

But black then played e himself:

Click Here To Show Diagram Code

[go]$$Bc
$$ ------------------
$$ | . . . . . . . . .
$$ | . . X . . . . . .
$$ | . . O X . 1 . . .
$$ | . . O , . . . . .
$$ | . . . O . . . . .
$$ | . . . . a . . . .
$$ | . . X . b . . . .
$$ | . . . . . . . . .
$$ | . . . c . . . . .
$$ | . . d . . . . . .[/go]

Now if white plays a, black can choose between b, c and d. Either way: How can white attack the top group in the following position(?):

Click Here To Show Diagram Code

[go]$$Bc
$$ ------------------
$$ | . . . . . . . . .
$$ | . . X . . . . . .
$$ | . . O X . X . . .
$$ | . . O , . . . . .
$$ | . . . O . . . . .
$$ | . . . . O . . . .
$$ | . . X . . . . . .
$$ | . . . . . . . . .
$$ | . . . X . . . . .
$$ | . . . . . . . . .[/go]

Seems hard to refute the deviation from the joseki at my level.
There is a pro game with a similar position, but in more than one way it is different:
http://eidogo.com/#2z4Nrt:0,29

B18 is a temporary attack on the upper side group. If Black defends the top, White attacks the left side.

Yes, b18 is sente now and leaves Black's top group over concentrated. Black wishes stone were further out at B or he played A instead so b18 wasn't sente and could play a more efficient move such as g16. And like Robert said "If Black defends the top, White attacks the left side."