If I make a plot with e.g.

import numpy as np
import matplotlib.pyplot as plt

image = np.random.random([256, 256])

fig = plt.figure()
ax = fig.add_axes([0.1, 0.1, 0.4, 0.4])
im = ax.imshow(image, cmap=plt.cm.gist_heat)
fig.colorbar(im)
fig.savefig('test.pdf')

The colorbar can contain white horizontal lines:

This actually becomes much more obvious when printing, as this can cause some kind of resonant/ringing pattern (I can send a photo of a printout if needed), and really makes the printed colorbar look bad.