This slip-up is in one of the much later episodes when the family goes to the theme park for Mike's meeting and Jan's poster gets mixed up with Mike's blueprints. When they have found the Yogi Bear poster and each family member is taking turns running to Mike, it is sunny (in fact, its sunny the whole episode). When Carol has the poster and is running towards Mike it is sunny, but when they cut to a shot of Mr. Brady walking out of the building it is raining. (I spotted this a long time ago and tried to enter it for a "classic TV rewind" but they rejected it, then used it on a Pop up Brady.)