Deliberate mistake: When Wayne begins playing the "Excalibur" guitar, the clerk stops him and directs him to a "No Stairway to Heaven" sign. The problem is that the guitar riff Wayne started playing sounded nothing like "Stairway to Heaven" so the clerk had no reason to interrupt him. Evidently the intro to "Stairway" was actually played in the theatrical version of the film, but its exclusion in the home video release makes the scene rather dubious.
Phaneron
Answer: You presumably haven't seen Terminator 2. The actor playing the cop is Robert Patrick, who played the shape-shifting T-1000 in that movie, who masqueraded as a policeman for most of the film. The T-1000's mission was to track down and terminate a boy - hence the "have you seen this boy" line that he says to Wayne.
Tailkinker