the following code produces syntax error:
>>> (x, y) += 1
While this is documented in
http://docs.python.org/3.0/reference/simple_stmts.html#augmented-assignment-statements
("An augmented assignment evaluates the target (which, unlike normal
аssignment statements, cannot be an unpacking)..."),
the syntax specification doesn't reflect this:
http://docs.python.org/3.0/reference/simple_stmts.html#grammar-token-augmented_assignment_stmt
http://docs.python.org/3.0/reference/grammar.html

Extract of the grammar:
  target          ::=  identifier | ...
  target_list     ::=  target ("," target)* [","]
  assignment_stmt ::=  (target_list "=")+ ...
  augmented_assignment_stmt ::=  target augop ...
  augop                     ::=  "+=" | ...

So there is only one possible target for "... += ..." and multiple 
targets for "... = ..".

Yes, but
target ::= identifier | "(" target_list ")" | ...
so (x,y) seems to be a valid target.

Oh yes! I didn't see "(" target_list ")" | "[" target_list "]", sorry.

The grammar in the doc is not the one used to generate the
parser/compiler.  The former is meant to be easier for humans to read,
the latter easier for the parser generator.  Neither completely embody
Python's syntax rules.  Additional restrictions may be applied later in
the chain.  From:
http://svn.python.org/view/python/trunk/Grammar/Grammar?rev=65872&view=auto

expr_stmt: testlist (augassign (yield_expr|testlist) |
                     ('=' (yield_expr|testlist))*)

augassign: ('+=' | '-=' | '*=' | '/=' | '%=' | '&=' | '|=' | '^=' |
            '<<=' | '>>=' | '**=' | '//=')
# For normal assignments, additional restrictions enforced by the
interpreter

Perhaps that should say "For augmented assignments....

Except maybe for the comment above, I think this issue can be closed.

Fixed in r68725.