1. MUTATION: docstring claims "does not mutate inputs" but the function mutates `default_prefs`. Line `result = default_prefs` creates an alias rather than a copy, so subsequent modifications via `result[key] = value` directly modify the caller's original dict. Demonstration: `defaults = {'a': 1, 'b': 2}; merge_user_prefs(defaults, {'b': 3}); defaults == {'a': 1, 'b': 3}` — the input was mutated.