In a perfect example of finding problems where none exist, Lucas decided it didnt make a great deal of sense for R2 to hide in a barely enclosed cave when Obi-Wan finds Luke laid out by the sandpeople, and so for the recent Blu-ray release he digitally added in some rocks, behind which R2 is tucked. How the hell everyone's favorite trash receptacle was able to maneuver around/behind those rocks in the first place is never explained; also not explained is what happens to those rocks, which disappear in the very next shot!