Three of "The Brady Bunch" kids have appeared on the show: Jan, Greg and Peter. Eve Plumb (Jan) played Jackie's mom in this episode only, and both Barry Williams (Greg) and Christopher Knight (Peter) played a gay couple who move in next door to the Formans in the episode, "We Will Rock You" (s8e18).
The first episode in which Eric is called "Foreplay".
Eve Plumb (The Brady Bunch (1969)) appears as Jackie's mother, a successful businesswoman. Brooke Shields later takes over the role during season six, and the character is changed to be more like an "older version" of Jackie.
Second episode in a row with a "one-off" appearance. Here, Eve Plumb makes her only appearance as Mrs Burkhart. In the episode immediately preceding, Amanda Fuller made her only appearance, as Donna's younger sister, Tina.
Despite what modern audiences might think, vendors in the 1970s rarely IDed buyers, speaking from experience.