Other mistake: Just after Bobby and Cindy get lost, there are shots showing Mike and Carol calling for them. A couple of minutes later the same shots are shown again, but this time they're flipped. (00:22:00)
Continuity mistake: At home, before leaving for the Grand Canyon, the trailer was much taller than the car, and looked more like a motor home. After that, it becomes a different vehicle more like a trailer, about the same height as the car. When the prosecutor steals the car, it's the tall version, and when they arrive at the Grand Canyon to look at the view, there's nothing attached to the car bumper. When they leave the Grand Canyon it's the shorter version, back to being a trailer.
Answer: Much trivia has been written about "The Brady Bunch, " including the various interrelationships and dynamics among the members of the cast. For example, much has been written about why Robert Reed's Mike Brady did not appear in a couple of episodes, including the series finale, due to rancorous creative and artistic differences with series creator, Sherwood Schwartz. However, in contrast, I have never run across any reasons given why a particular child did not appear in specific episodes - only that the episodes were missed. This suggests the causes were likely unexceptional, such as illness, injuries, vacations, or real-life family obligations.
Michael Albert
Although those options ARE possible reasons, it just seems a bit ironic that this happened all in Season 2 ONLY and within a short amount of nearly consecutive episodes. Never happened during the other 5 years of the show at all.