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)
Other mistake: When Jan finds the picture of her Aunt Jenny, we see a close-up of the picture with her hand holding it. She then passes it to Carol - and the same shot of the picture with the hand is shown again. (00:02:10)
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.