Parser/parsetok.c was recently changed (e.g. http://hg.python.org/cpython/rev/2bd7f40108b4 ) to raise an error if multiple statements were found in a single-statement compile call.  It sensibly ignores trailing whitespace and comments.  Unfortunately,

(1)  It looks only at (c == ' ' || c == '\t' || c == '\n' || c == '\014') as opposed to using Py_UNICODE_ISSPACE(ch)
(2)  It assumes that a "#" means the rest of the line is OK, instead of looking for additional linebreaks.

Not sure whether to mark this a bug or an enhancement, since it is already strictly better than the 3.2 behavior of never warning about extra text.

New changeset 00c4efbf57c3 by Benjamin Peterson in branch 'default':
check after comments, too (#13832)
http://hg.python.org/cpython/rev/00c4efbf57c3

The tokenizer doesn't consider unicode spaces, either.

Ignoring non-ascii whitespace is defensible, and I agree that it should match the rest of the parser.  Ignoring 2nd lines is still a problem, and supposedly part of what got fixed.  Test case:

s="""x=5  # comment
x=6
"""
compile(s, "<testbadsingle>", 'single')

$ ./python 
Python 3.3.0a0 (default:50a4af2ca654+, Jan 20 2012, 10:59:48) 
[GCC 4.5.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> s="""x=5  # comment
... x=6
... """
>>> compile(s, "<blah>", "single")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<blah>", line 1
    x=5  # comment
                 ^
SyntaxError: multiple statements found while compiling a single statement