I close third storey sash windows with mine, I just make sure that I'm careful. I always try to push straight with the pole, exerting force upwards on the pole not sideways.
A trick I use on side opening windows is to 'catch' the open bottom corner of the window frame on the middle of the brush face (in the middle of the bristles)and then push the pole upwards to push the window shut, it is much better than just pushing it shut with a sideways movement.