Not reading back clearances in the correct order.
ie. "After the landing 747, cross runway 27L"
Rather than the more usual "Cross runway 27L......." Cue mucho panic ".......after the landing 747."
Arrrrrgh! One day somebody's just going to readback "Cross runway 27L" and the ATCO will think he/she's heard the condition as well.
Gonzo.