• #9819 – ENH: Add order parameter validation to hannan_rissanen (merged Jun 2026) Implemented a new _validate_order_params helper to catch invalid AR/MA/initial-order combinations in the Hannan-Rissanen ARMA estimator, turning two long-standing xfail tests into passing ones. Resolves #9817.

• #25366 – MAINT: sparse: improve save_npz error message for unsupported formats (merged Jun 2026) Improved the NotImplementedError raised by save_npz for LIL/DOK sparse matrices to clearly suggest converting via .tocsr() or .tocoo() first, and extended tests to cover both matrix and array variants. Resolves #25305.

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