Continuity mistake: As Darren passes the cheese touch to the girl, she turns around in shock. In the next shot, she is standing still with everyone crowded around her running away, and Darren is no longer seen.
Other mistake: When Gregory has read the note from Fregly, he realises he put his thumb on a booger. When we see Greg on the floor, the note should be nearby. Instead it is several feet from him, in line with the bin. This is strange as Greg only dropped it and the note didn't flutter over there as Greg only turned when he hit the floor. In fact, the movement should've moved it closer to him. In line with the bin is even weirder.
Continuity mistake: When Greg and Rowley were sprayed with the milk like substance at Halloween by the 3 thugs in the red pick up, they ran off. Shortly after, their costumes and Rowley's shield showed no traces at all of the milk like substance.
Continuity mistake: As Greg walks down the stairs exiting his first day at middle school a black girl walks out of the entrance 4 times as Rowley says "hey Greg, wanna come over and play?" (00:15:50)
Continuity mistake: In the scene where Rodrick caught Greg and Rowley in his room, Rowley clings to Rodrick's left leg but when they switch to the wide angle Rowley is holding on to Rodrick's right leg. The angle switches again and Rowley is back on his left leg. (00:20:35)
Continuity mistake: When it is announced that Rowley has won the school cartoonist job, Brett (at the desk next to Rowley's left), starts to clap, then in the very next shot his arms are on the table and you can see him start to clap again.
Revealing mistake: When we see the man in the green shirt breakdancing, we can obviously tell that he is a professional breakdancer and not the same guy as the one breakdancing. The professional breakdancer is much thinner and reveals his face, which is completely different from the original man in the green shirt. (04:32:33 - 04:34:23)
Answer: That was not him singing, that was another kid singing.
Turangaa_Maxx