From Lessons in the Fundamentals of Go, by Toshiro Kageyama:
Following the standard handicap opening of 1, 3, and 5 the joseki runs from Black 6 to White 27. If Black now descends to 'A', White will live easily. That is how most people would play, but there is a move here that will vex White considerably. Can you find Black 28 and read out the whole continuation?