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)
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.