stage-2 notebook says:
for i in range(20):
    row0 = i*250
    row251 = row0+250
    img = df.iloc[row0:row251, :]
    fig, ax = plt.subplots()
    plt.imshow(img, cmap='jet')
i think it should have been
row251 = row0+251
the plots should in the notebook has spiky boundary conditions at the edge which i think is wrong due to the bug.