Continuity mistake: When Joe goes to quit, it is late afternoon (it is getting dusky). But when he shows up at Ben's house afterwards to tell him that he just quit, it is lighter, like it's the middle of the day.
Audio problem: Near the end of the film, Joe's girlfriend suggests that he stays as Orgazmo - her lips don't match what she is saying.
Visible crew/equipment: When Lisa and Joe are kneeling on the floor in his apartment, just after Lisa arrived, in the window behind Joe's head, you can see a camera and what may be crew members in the reflection.
Deliberate mistake: The shot of Orbison's house buring at the end is quite obviously fake. You can see that it's made of cardboard and paper, and even the windows burn and curl like normal bits of paper do when set alight.
Chosen answer: No, Happy Tarts is not a real product (unless you live in South Park where Cartman sometimes has them along with Cheesy Poofs and Snacky Cakes).
Myridon