# Buggy code
def __init__(
    self, data=None, index=None, dtype=None, name=None, copy=False, fastpath=False
):
    # we are called internally, so short-circuit
    if fastpath:
        # data is an ndarray, index is defined
        if not isinstance(data, SingleBlockManager):
            data = SingleBlockManager(data, index, fastpath=True)
        if copy:
            data = data.copy()
        if index is None:
            index = data.index

    else:
        if index is not None:
            index = _ensure_index(index)

        if data is None:
            data = {}
        if dtype is not None:
            dtype = self._validate_dtype(dtype)

        if isinstance(data, MultiIndex):
            raise NotImplementedError(
                "initializing a Series from a MultiIndex is not supported"
            )
        elif isinstance(data, Index):
            # need to copy to avoid aliasing issues
            if name is None:
                name = data.name

            data = data._to_embed(keep_tz=True)
            copy = True
        elif isinstance(data, np.ndarray):
            pass
        elif isinstance(data, Series):
            if name is None:
                name = data.name
            if index is None:
                index = data.index
            else:
                data = data.reindex(index, copy=copy)
            data = data._data
        elif isinstance(data, dict):
            if index is None:
                if isinstance(data, OrderedDict):
                    index = Index(data)
                else:
                    index = Index(_try_sort(data))
            try:
                if isinstance(index, DatetimeIndex):
                    if len(data):
                        # coerce back to datetime objects for lookup
                        data = _dict_compat(data)
                        data = lib.fast_multiget(
                            data, index.astype("O"), default=np.nan
                        )
                    else:
                        data = np.nan
                # GH #12169
                elif isinstance(index, (PeriodIndex, TimedeltaIndex)):
                    data = [data.get(i, nan) for i in index] if data else np.nan
                else:
                    data = lib.fast_multiget(data, index.values, default=np.nan)
            except TypeError:
                data = [data.get(i, nan) for i in index] if data else np.nan

        elif isinstance(data, SingleBlockManager):
            if index is None:
                index = data.index
            else:
                data = data.reindex(index, copy=copy)
        elif isinstance(data, Categorical):
            if dtype is not None:
                raise ValueError("cannot specify a dtype with a Categorical")
        elif isinstance(data, types.GeneratorType) or (
            compat.PY3 and isinstance(data, map)
        ):
            data = list(data)
        elif isinstance(data, (set, frozenset)):
            raise TypeError("{0!r} type is unordered".format(data.__class__.__name__))
        else:
            # handle sparse passed here (and force conversion)
            if isinstance(data, ABCSparseArray):
                data = data.to_dense()

        if index is None:
            if not is_list_like(data):
                data = [data]
            index = _default_index(len(data))

        # create/copy the manager
        if isinstance(data, SingleBlockManager):
            if dtype is not None:
                data = data.astype(dtype=dtype, raise_on_error=False)
            elif copy:
                data = data.copy()
        else:
            data = _sanitize_array(data, index, dtype, copy, raise_cast_failure=True)

            data = SingleBlockManager(data, index, fastpath=True)

    generic.NDFrame.__init__(self, data, fastpath=True)

    object.__setattr__(self, "name", name)
    self._set_axis(0, index, fastpath=True)


# Traceback (ValueError)
In [37]: pd.Series(pd.Categorical([1,2,3]), dtype='category')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-37-87a1a3228158> in <module>()
----> 1 pd.Series(pd.Categorical([1,2,3]), dtype='category')

ValueError: cannot specify a dtype with a Categorical

# Fixed code
def __init__(
    self, data=None, index=None, dtype=None, name=None, copy=False, fastpath=False
):
    # we are called internally, so short-circuit
    if fastpath:
        # data is an ndarray, index is defined
        if not isinstance(data, SingleBlockManager):
            data = SingleBlockManager(data, index, fastpath=True)
        if copy:
            data = data.copy()
        if index is None:
            index = data.index

    else:
        if index is not None:
            index = _ensure_index(index)

        if data is None:
            data = {}
        if dtype is not None:
            dtype = self._validate_dtype(dtype)

        if isinstance(data, MultiIndex):
            raise NotImplementedError(
                "initializing a Series from a MultiIndex is not supported"
            )
        elif isinstance(data, Index):
            # need to copy to avoid aliasing issues
            if name is None:
                name = data.name

            data = data._to_embed(keep_tz=True)
            copy = True
        elif isinstance(data, np.ndarray):
            pass
        elif isinstance(data, Series):
            if name is None:
                name = data.name
            if index is None:
                index = data.index
            else:
                data = data.reindex(index, copy=copy)
            data = data._data
        elif isinstance(data, dict):
            if index is None:
                if isinstance(data, OrderedDict):
                    index = Index(data)
                else:
                    index = Index(_try_sort(data))
            try:
                if isinstance(index, DatetimeIndex):
                    if len(data):
                        # coerce back to datetime objects for lookup
                        data = _dict_compat(data)
                        data = lib.fast_multiget(
                            data, index.astype("O"), default=np.nan
                        )
                    else:
                        data = np.nan
                # GH #12169
                elif isinstance(index, (PeriodIndex, TimedeltaIndex)):
                    data = [data.get(i, nan) for i in index] if data else np.nan
                else:
                    data = lib.fast_multiget(data, index.values, default=np.nan)
            except TypeError:
                data = [data.get(i, nan) for i in index] if data else np.nan

        elif isinstance(data, SingleBlockManager):
            if index is None:
                index = data.index
            else:
                data = data.reindex(index, copy=copy)
        elif isinstance(data, Categorical):
            # GH12574: Allow dtype=category only, otherwise error
            if (dtype is not None) and not is_categorical_dtype(dtype):
                raise ValueError(
                    "cannot specify a dtype with a Categorical unless dtype='category'"
                )
        elif isinstance(data, types.GeneratorType) or (
            compat.PY3 and isinstance(data, map)
        ):
            data = list(data)
        elif isinstance(data, (set, frozenset)):
            raise TypeError("{0!r} type is unordered".format(data.__class__.__name__))
        else:
            # handle sparse passed here (and force conversion)
            if isinstance(data, ABCSparseArray):
                data = data.to_dense()

        if index is None:
            if not is_list_like(data):
                data = [data]
            index = _default_index(len(data))

        # create/copy the manager
        if isinstance(data, SingleBlockManager):
            if dtype is not None:
                data = data.astype(dtype=dtype, raise_on_error=False)
            elif copy:
                data = data.copy()
        else:
            data = _sanitize_array(data, index, dtype, copy, raise_cast_failure=True)

            data = SingleBlockManager(data, index, fastpath=True)

    generic.NDFrame.__init__(self, data, fastpath=True)

    object.__setattr__(self, "name", name)
    self._set_axis(0, index, fastpath=True)