When Kim first meets Monique, Monique is wearing a green top, but at the register, Monique is wearing a red top.
When Jackie destroys the Pain King banner in the crowd, the man in the bottom-right corner disappears for about 1 frame right as the laser hits the banner.
Single-Frame Anomalies of this sort are part of the Persistence of Vision Animation Process, often known as "animation blur", and strictly speaking not Errors.
In the coffee house scene, Kim has a spoon in her cup, but when she gets up, the spoon is gone.
After the Jackal throws Pain King and Steel Toe into one another and turns to address the crowd, as he is talking, his mouth momentarily disappears behind his neck
Up until the point where Jackie gives Kim the tickets to see GWA, Jackie's eyes are white with black pupils. For the rest of the episode, his eyes are just plain black dots.
Since Monique just moved to Middleton, why would she get a job at Club Banana before even starting school there?
Perhaps she already was an employee in her old city and simply transfered stores.
The transfer may be the reason she moved to Middleton.
After Jackie the Jackal threw Ron against the wall, Rufus left Ron's pocket and hid under some lettuce or something green right beside Ron. Jackie the Jackal, then threw Ron into the ring, which is quite a long way away from where they were at the time. Some time later, Ron throws Rufus so he could get the medallion off of Jackie the Jackal. How did Rufus make his way back to Ron? He should still be in the hall, or at least shown making his way back to Ron.
Not being seen making their ways back to each other from a reasonable distance is not an Error, it was just unseen. Had Rufus been left home and then showed up, that would be an Error.
The GWA is an imitation of the WWE, a professional wrestling organization. The voices of Steel Toe and Pain King are Bill Goldberg (for the old WCW) and Andrew Martin (A.K.A. Test from the WWE), respectively.