Not the OP, but I think what they are referring to is the following:

  def foo():
    return bar
  
  if ...:
    bar = 42
    print(foo())
  else:
    print(foo())

If ... evaluates to a truthy value, bar will be defined and foo() will return 42, if it is falsy, bar will not be defined and the code will throw a NameError.

I would agree that this is extremely confusing behaviour. Of course people wouldn't write code like this (hopefully), but a similar situation might occur in a much more obscure situation.