How can using Python backslash line continuation be subtly wrong?

Question

This section of the Python docs describes a subtle error that can happen when using backslash line continuation, if you accidentally add a space after the backslash.

It says, the following would be a syntax error (with a space after the backslash):

if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \ 
   calculate_number(10, 20) != forbulate(500, 360):
      pass

...but that the next snippet would just be subtly wrong (with a space after the backslash):

value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \ 
        + calculate_number(10, 20)*forbulate(500, 360)

I tried to reproduce this with this:

value = 1 + 2 \ 
        + 3 + 4

but got this:

  File "foo.py", line 1
    value = 1 + 2 \
                   ^
SyntaxError: unexpected character after line continuation character

Under what circumstances would that accidentally-added space not be a syntax error?