The Baker Street graffiti I.O.U. is the one people seem to have missed most frequently. I expect it’s because they didn’t focus on it as directly as the apple and the windows. They did show it twice to make up for that, though—during the arrest scene and again when John returned to Baker Street the next morning.
I showed all of the I.O.U.s in this post, but here are some additional screenshots of the Baker Street graffiti for you: