Why would I ever risk inflicting a stack trace like

  Traceback (most recent call last):
    File "x.py", line 2, in <module>
      foo.encode()
  UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 0: ordinal not in range(128)

on a user of Python 3.x where it isn't possible? (Note the UnicodeDecodeError coming from an attempt to encode.)