Mistake in South Park

Okey, this is not accturly a mistake, but it's stupid. Season 1 episode 6. Stan's grandfather is 102, and they cellebrating it. First the cake is on the table, and then one of the people talking and they focused on the face. And when they take the camera back again, the cake is suddenly gone AND nobody had left the table or moved. So somebody have just conjured it away or something. 
And then suddely the cake is back on the table.