Calling pyplot.figure() multiple times causes a partial trace to appear in the terminal (partial because the entire chain of calls is not shown):

Traceback (most recent call last):
File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt4.py", line 156, in
lambda: self.close_event())
File "/usr/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 1564, in close_event
self.callbacks.process(s, event)
AttributeError: 'FigureCanvasQTAgg' object has no attribute 'callbacks'

This bug was reported on Nabble in May:

http://matplotlib.1069221.n5.nabble.com/Qt-close-bug-revisited-td35203.html

The first user to report this bug presented a simple bit of code for reproducing the bug:

import matplotlib.pyplot as plt
fig = plt.figure()
plt.plot([0, 1])
fig = plt.figure()
plt.plot([0, 1])

I am able to get the same error message using the same code.

python2-matplotlib 1.1.1-1
python2 2.7.3-2
archlinux 64 bit

-Mike