The trouble with backpacks and trolleys is that you are going to be more careful about using water. Thats only natural.
However, using too much water can cause problems as well. Water from one window will run down the brick work and/or drip onto the window below. This can continue for a long time. How often have you had water dripping onto you from above as you clean the downstairs windows? So after you have finished the dowstairs windows that water is still dripping from above and who knows what sort of impurities it has picked up on the way to leave on your nice clean window.
