1. MUTATION: docstring states "Does not mutate inputs" but the function mutates `default_prefs` by assigning to `result[key]`. Since `result = default_prefs` creates a reference (not a copy), modifications to `result` are modifications to the original dict. Demonstration: `defaults = {'a': 1}; merge_user_prefs(defaults, {'a': 2}); defaults == {'a': 2}` — the caller's dict was mutated.