When the boys are looking at the weapons on the counter, the red nuncukas under the yellow ones keep disappearing when the camera angle changes.
One person refers to Cartman as "that fat little eight year old boy" at the end after Cartman inadvertently flashes the audience. However, the boys would be nine and in fourth grade ever since the middle of the fourth season. A possible explanation for this could be the same as the explanation for the episode's resolution: Many of the adults in South Park are relatively unaware of what's going on with their children's private lives or simply aren't concerned.
Kyle's anime counterpart has black hair, whereas he really has auburn hair, as shown in How to Eat With Your Butt. However, this may be excusable because the boys are imagining their ideal selves and Kyle doesn't like his hair.
After Stan tries to pick out the ninja star with a pair of pliers, we see all four of the boys and Kyle tells Stan to "Stop dude! You're gonna scramble his brain!" If you look on the far right on the snowy tree closest to Kyle, we can see a black line in the shape of Butters eyebrow, If you watch closely, it moves exactly like the eyebrow on Butters when he moves his head.
When Craig, Token, Jimmy, Clyde, Cartman, Kenny, Kyle and Stan are fighting in anime, you can clearly see Jimmy jump. He should not be able to do this since he is crippled but could've done this using his hands for support, suggesting he has a level of enablibility and intelligence. It could also be a similar circumstance where Kyle imagines his anime counterpart with black hair instead of his natural curly red color-It could just be an imagination of their ideal selves.
When The boys are walking away from Butters (after they told him he can't be a ninja), the red cloth on Cartman's right arm isn't there temporarily.
Stan already knew who professor chaos/butters is, he learnt that secret on season 6 My future self 'n' me but still seemed confused on who is him.
At the end in the community center, you can see Randy and Sharon behind a man to the left. When the man stands up Randy and Sharon have moved to the right, with the man being in between them. When it switches to most of the people again, it goes back to normal.