The window layout manager is smart enough not to let you push the window totally off the screen, but if you grab a window and pull it down under the taskbar (assuming the taskbar is in the default bottom position and does not autohide) in Windows 7, you're somewhat boned because while the window is still there, the taskbar's zorder makes it impossible to click on it to recover it. You can fix this situation by unlocking the taskbar (if it is locked) and moving it or by setting it to autohide, but neither of these things is a particularly intuitive move for the average user, I'd guess.