This slip up occurs during season 3, episode 1, titled "Listen To The Rain On The Roof".

After Mary Alice begins her usual storyline, watch the part about Edie Brit putting up the for sale sign on Mary Alice's house. Look behind Edie, at the rain. The rain is even all through the shot, except behind her it's heavier and obviously the work of a hose or some sort of "super soaker". The rain is completely concentrated in that part only, next to it there is hardly any rain at all, and on the other side, it's misty rain.