You can sometimes, but obviously you have to be careful.
Ordinary windows are not much of a problem, but sash windows are a right pain

I can cope fine with a normal window being open, even when you can't nudge them shut you can still clean them, but with sash windows you gotta be soooo careful!
Last summer I was doing a 3 storey office in our town centre, they have modern, aluminium windows that are sash style in the way they open.
No one ever leaves a window open...well they never did in the past

Deep window sills and its very early in the morning, couldn't see the lower pane...cleaned top pane, dropped to lower pane and brush disappears from sight

Then you panic trying to whip the blasted thing out of the window and of course the brush gets stuck, eventually the brain cuts in and you cut the water flow

I have absolutely no idea where the water went or what I soaked, I just prayed that whatever got wet dried out before the office staff got in later...it must have been ok cos no one ever said anything!
I check very carefully now that windows are shut, or if they are open I try not to clean a window pane that isn't there!
Ian