A list of syntax/indentation errors I've encountered in CPython.

# Python/compile.c
    "'continue' not properly in loop"
    "'continue' not supported inside 'finally' clause"  # Until loop
    "default 'except:' must be last"
    "from __future__ imports must occur at the beginning of the file"
    "'return' outside function"
    "'return' with value in async generator"
    "'break' outside loop"
    "two starred expressions in assignment"
    "asynchronous comprehension outside of an asynchronous function"
    "'yield' outside function"  # For both yield and yield from
    "'yield from' inside async function"
    "'await' outside function"
    "'await' outside async function"
    "starred assignment target must be in a list or tuple"
    "can't use starred expression here"
    "too many statically nested blocks"  # Max. 20
    # This is one of the few places in the cpython code base that I really
    # don't understand. It feels a bit hacky if you look at the implementation
    # of UNPACK_EX.
    "too many expressions in star-unpacking assignment"

    # Just ignore this one, newer versions will not be affected anymore and
    # it's a limit of 2^16 - 1.
    "too many annotations" # Only python 3.0 - 3.5, 3.6 is not affected.

# Python/ast.c
    # used with_item exprlist expr_stmt
    "can't %s %s" % ("assign to" or "delete",
            "lambda"
            "function call"  # foo()
            "generator expression"
            "list comprehension"
            "set comprehension"
            "dict comprehension"
            "keyword"
            "Ellipsis"
            "comparison"
        Dict: Set: Num: Str: Bytes: JoinedStr: FormattedValue:
            "literal"
        BoolOp: BinOp: UnaryOp:
            "operator"
        Yield: YieldFrom:
            "yield expression"
        Await:
            "await expression"
        IfExp:
            "conditional expression"
    "assignment to keyword"  # (keywords + __debug__)  # None = 2
    "named arguments must follow bare *"  # def foo(*): pass
    "non-default argument follows default argument" # def f(x=3, y): pass
    "iterable unpacking cannot be used in comprehension"  # [*[] for a in [1]]
    "dict unpacking cannot be used in dict comprehension"  # {**{} for a in [1]}
    "Generator expression must be parenthesized if not sole argument"  # foo(x for x in [], b)
    "positional argument follows keyword argument unpacking" # f(**x, y) >= 3.5
    "positional argument follows keyword argument"  # f(x=2, y) >= 3.5
    "iterable argument unpacking follows keyword argument unpacking" # foo(**kwargs, *args)
    "lambda cannot contain assignment"  # f(lambda: 1=1)
    "keyword can't be an expression" # f(+x=1)
    "keyword argument repeated"  # f(x=1, x=2)
    "illegal expression for augmented assignment"  # x, y += 1
    "only single target (not list) can be annotated" # [x, y]: int
    "only single target (not tuple) can be annotated" # x, y: str
    "illegal target for annotation" # True: 1`
    "trailing comma not allowed without surrounding parentheses" # from foo import a,
    "bytes can only contain ASCII literal characters." # b'ä'  # prob. only python 3
    "cannot mix bytes and nonbytes literals" # 's' b''
    "assignment to yield expression not possible"  # x = yield 1 = 3

    "f-string: empty expression not allowed"  # f'{}'
    "f-string: single '}' is not allowed"  # f'}'
    "f-string: expressions nested too deeply"  # f'{1:{5:{3}}}'
    "f-string expression part cannot include a backslash"  # f'{"\"}' or f'{"\\"}'
    "f-string expression part cannot include '#'"  # f'{#}'
    "f-string: unterminated string"  # f'{"}'
    "f-string: mismatched '(', '{', or '['"
    "f-string: invalid conversion character: expected 's', 'r', or 'a'" # f'{1!b}'
    "f-string: unexpected end of string"  # Doesn't really happen?!
    "f-string: expecting '}'"  # f'{'
    "(unicode error) unknown error
    "(value error) unknown error
    "(unicode error) MESSAGE
        MESSAGES = {
            "\\ at end of string"
            "truncated \\xXX escape"
            "truncated \\uXXXX escape"
            "truncated \\UXXXXXXXX escape"
            "illegal Unicode character"       # '\Uffffffff'
            "malformed \\N character escape"  # '\N{}'
            "unknown Unicode character name"  # '\N{foo}'
        }
    "(value error) MESSAGE  # bytes
        MESSAGES = {
            "Trailing \\ in string"
            "invalid \\x escape at position %d"
        }

    "invalid escape sequence \\%c"  # Only happens when used in `python -W error`
    "unexpected node"  # Probably irrelevant
    "Unexpected node-type in from-import"  # Irrelevant, doesn't happen.
    "malformed 'try' statement"  # Irrelevant, doesn't happen.

# Python/symtable.c
    "duplicate argument '%U' in function definition"
    "name '%U' is assigned to before global declaration"
    "name '%U' is assigned to before nonlocal declaration"
    "name '%U' is used prior to global declaration"
    "name '%U' is used prior to nonlocal declaration"
    "annotated name '%U' can't be global"
    "annotated name '%U' can't be nonlocal"
    "import * only allowed at module level"

    "name '%U' is parameter and global",
    "name '%U' is nonlocal and global",
    "name '%U' is parameter and nonlocal",

    "nonlocal declaration not allowed at module level");
    "no binding for nonlocal '%U' found",
    # RecursionError. Not handled. For all human written code, this is probably
    # not an issue. eval("()"*x) with x>=2998 for example fails, but that's
    # more than 2000 executions on one line.
    "maximum recursion depth exceeded during compilation");

# Python/future.c
    "not a chance"
    "future feature %.100s is not defined"
    "from __future__ imports must occur at the beginning of the file"  # Also in compile.c

# Parser/tokenizer.c
    # All the following issues seem to be irrelevant for parso, because the
    # encoding stuff is done before it reaches the tokenizer. It's already
    # unicode at that point.
    "encoding problem: %s"
    "encoding problem: %s with BOM"
    "Non-UTF-8 code starting with '\\x%.2x' in file %U on line %i, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details"

# Parser/pythonrun.c
    E_SYNTAX: "invalid syntax"
    E_LINECONT: "unexpected character after line continuation character"
    E_IDENTIFIER: "invalid character in identifier"
    # Also just use 'invalid syntax'. Happens mostly with stuff like `(`. This
    # message doesn't really help the user, because it only appears very
    # randomly, e.g. `(or` wouldn't yield this error.
    E_EOF: "unexpected EOF while parsing"
    # Even in 3.6 this is implemented kind of shaky. Not implemented, I think
    # cPython needs to fix this one first.
    # e.g. `ast.parse('def x():\n\t if 1:\n \t \tpass')` works :/
    E_TABSPACE: "inconsistent use of tabs and spaces in indentation"
    # Ignored, just shown as "invalid syntax". The error has mostly to do with
    # numbers like 0b2 everywhere or 1.6_ in Python3.6.
    E_TOKEN: "invalid token"
    E_EOFS: "EOF while scanning triple-quoted string literal"
    E_EOLS: "EOL while scanning string literal"

    # IndentationError
    E_DEDENT: "unindent does not match any outer indentation level"
    E_TOODEEP: "too many levels of indentation"  # 100 levels
    E_SYNTAX: "expected an indented block"
              "unexpected indent"
              # I don't think this actually ever happens.
              "unexpected unindent"


    # Irrelevant for parso for now.
    E_OVERFLOW: "expression too long"
    E_DECODE: "unknown decode error"
    E_BADSINGLE: "multiple statements found while compiling a single statement"


Version specific:
Python 3.5:
    'yield' inside async function
Python 3.3/3.4:
    can use starred expression only as assignment target
