On a histogram with a single value repeated for 100 times I get an extra tiny box.
The code to reproduce the issue:

from matplotlib import pylab as plt
plt.hist([33]*100)
plt.show()

Here you can see a black line next to 32.6.

• Matplolib: 1.5.1-1ubuntu1 (from repository), Python 2.7.12, Ubuntu 16.04.1
• The same problem can be reproduced in python 3.5.2.