The crew crossed the runway at number W11, in stead of continuing to taxi all around the runway at the south end, using taxiway Zulu, as they had been cleared to do by the traffic controller.

This certainly was not the intention of the crew. But what they found perhaps confusing, was that the tower controller cleared them "to cross 36C at Zulu". Although the wording "Zulu" could not possibly be misunderstood about where they were supposed to be (Z->Z2->Z1->Z), it was the word "cross" that confused them. To them, this meant they had to physically be on the runway to "cross" it. But in correct ICAO language, the word "cross" is also used when the aircraft needs to taxi around the south end of this runway and needs to cross the holding line at Z2. 

In a telephone discussion between the controller and the captain, the controller pointed out to the captain that there was an extra sheet of paper in the airport diagrams, laying out this particular "hotspot" situation. Over the phone the controller heard papers shuffling and then the captain sigh "Oh.. .buggars... If I had seen that, I would have never made this mistake"

Many solutions have been tried to solve this. In the end, the aiport physically blocked both entries W12 and W11. End of problem.