The behavior of dict iteration has changed in Python 3.6, in that inserting keys during iteration has a different and unpredictable affect.  For this code:

d = {'foo': 1}
for key in d:
   print(key)
   d.pop(key)
   d[key] = 1

Python 3.5 prints a single 'foo' (one pass through the loop).  Python 3.6 generates five lines of 'foo' (five passes through the loop).  Of course this code is pathological, but I found this behavior from a bug in code where the pathology was a lot less obvious - see https://github.com/nipy/nipy/issues/420

This "bug" has always existed, its just that its manifestation has changed.  See issue 19332 for background.

To clarify from comments on issue 19332:

"""
* The normal rule (not just for Python) is that a data structures have undefined behavior for mutating while iterating, unless there is a specific guarantee
"""

The change in Python 3.6 is just a different undefined behavior.