Easy to reproduce:

import matplotlib.pyplot as plt
import matplotlib.transforms as mtrans

ax = plt.subplot(122, xscale='log')
t = mtrans.BlendedGenericTransform(ax.transData, ax.transAxes)
plt.plot(50, 0.5, 'ob', transform=t)
plt.show()

Manually resize the window, the point should stay in the middle of the axes, but stays static, as if the values given were in device coordinates.

Likely to be down to a caching issue. Related to #1247. Almost certainly caused by me.