Minimal example:

import matplotlib.pyplot as plt

plt.figure()
ax = plt.subplot()
c = plt.Circle((0, 0), 1, facecolor='none', alpha=1)
ax.add_artist(c)
plt.show()

This plots a black filled in circle. I expected facecolor='none' to override alpha=1 and set a transparent facecolor, but instead it seems that alpha overrides the facecolor argument.

Present on 2.0.0b4+2839.g631e079 (installed from source using pip on OSX)