While Kaep's fumble certainly did not help the team win on Sunday, I believe that for the long-term success of the team, it was the most important play of the day. Kaep carried the ball a bit carelessly and lost it deep in the opponent's territory. He looked dejected on the sideline right after.
Up to that point, Kaep has had nothing but success this year, in addition to plenty of media attention for a 2nd string QB. With the fumble, he got the opportunity to learn the importance of taking care of the ball, and the ups and downs that come with playing QB. There is no reason to say that he was full of himself, but he certainly got a serving of humble pie. Kaep will be a better QB of the future for this mistake.
The second aspect of this situation is the amazing job that the coaches are doing. I love that they predominantly sit rookies during game time. Rookies have their hands full and learn plenty from practices. If a rookie can contribute, they will play like Culliver did last year. The coaches are taking care not to rush anyone onto the field though. Now that Kaep sat for a year and worked hard in practice, the coaches are getting him on the field in limited action. This is Kaep's next learning step. And the fumble was a great learning experience without costing the team a victory.
Additionally, the coaches showed absolute trust in Kaep by putting him back in later in the game. I love this too. Harbaugh sees the best in every player, and the players respond by living up to the expectation that the coach sets. Worked for Kyle Williams, and I suppose for many other players.