The scene where Randy's drama teacher accidentally throws the flowers at Greg. Greg hands the flowers back to the teacher. In the next shot the flowers are in Randy's hand.
Greg is 1 month away from turning 15 and hoping to save money and buy a used car when he turns 16, which is "right around the corner." When he tells his parents, Carol exclaims, "I was worried enough when you got a bicycle!". But Carol and Mike got married during the series pilot, so unless Greg didn't get a bicycle until he was in his teens, Carol wasn't in the Bradys' lives when Greg got his first bike.
When Greg is trying to retrieve Mike's designs left in the back of Mr. Peterson's car, his daughter Randy says, "He dropped me off, and than went to the garage to have some work done." She never tells Greg which garage her father took the car for work.
When Mr. Peterson drives away, the shipping tube is sticking out above the tailgate. Greg could have easily reached out to grab it. Also, since the tailgate window was down, Mr. Peterson would have heard Greg yelling for him to stop.
During his second attempt at delivering the plans, Greg puts the shipping tube in the back of Mr. Peterson's station wagon, sticking out the back with the rear window down. Since Greg had already lost the plans once before, it doesn't make sense for him to not hold on to them a little more closely during his second attempt.