On the Poorly Planned part: Obi-wan Kenobi has been around R2D2 a good while in the first 2 films, and possibly the third movie more than likely. Yet there is no plausible reason why his character would forget the droid in Episode 4. He would know R2D2. This is the problem with doing prequels in general.
Ah yes, we all know how HONEST Obi-Wan Kenobi was in Ep. 4.......