Continuity mistake: When you look at the house from the outside the upstairs is on the left. But, when you go inside facing the same direction, the upstairs is on the right.
Deliberate mistake: In the Search for the Stars contest, at the end of the song their mouths have stopped singing as they go to bow but you continue to keep hearing them apparently sing a few seconds longer as it fades out. This is a joke referring to the original Brady Bunch episode where the kids sing the song on live television and the song similarly fades out after they've stopped singing.
Plot hole: Even though the Brady Bunch kids won $20,000 (check, not cash) in the "Search for the Stars" contest, they would not have immediate access to it, so would not be able to merely take the oversized representative check - which would be nonlegal tender - to pay for the taxes; they would have to wait until they received an authentic hard copy (real check) and wait until the check "cleared" the bank (was legitimate and payable). Clearing the check could take weeks because the Brady's didn't have at least $20,000 in their checking account to cover what could be a bogus check. (01:19:03)
Revealing mistake: During the scene where Mr. Dittmeyer is standing in front of the 9 TVs, you can see that the middle 3 people (Mike, Carol and Alice) are all taken from the same camera/angle as they all have the same background. As such that would then mean they were recordings being played back vs. being live shots as beginning of the musical number portrays.