Monk is wearing a green wristband pass for the concert on one arm, but one of Monk's many obsessions is symmetry. For example, he once hurt his hand in earlier episodes and he put bandages on both hands. There are many other occasions where Monk deals with symmetry and especially symmetry of human body and his own. Monk should be wearing two wristbands, one for each arm.
Natalie's shoes change during the episode. She is wearing heels when they are in the police station at the beginning of the episode, flats when they arrive at the rock concert, and tennis shoes at the end of the episode when they are trying to catch the beach ball.
When Monk, Natalie, & Capt Stottlemeyer first arrive at the festival, Monk begins to object to going in. At one point you can see a female extra in a tank top and glasses walk past Monk into the festival. In the next shot she can be seen bumping into him.
When they show the stage for the first time, note the hot blonde girl on the left in the yellow-striped halter and khaki short-shorts on the left. Then, a few seconds later when Monk is looking for the pay-phone, she is on the ground on screen right. After Monk makes his "phone call", you see her back on the stage.
Various extras in this episode are seen multiple times throughout the episode. Though this could be done, there is one extra, the large black woman, who first appears at Monk's arrival hanging out with several others, and then shortly after we see Monk at the port-a-potty for the first time.
When Monk is in the parking lot admonishing the concert goers for dancing there, there are a number of dancers hoisting and drinking beer from bottles. All of the bottles that you see still have their caps on, even though the dancers are "drinking" from them.