1. MUTATION: docstring claims "does not mutate inputs" but the function mutates `default_prefs`. Line 3 assigns `result = default_prefs` without copying, so all subsequent mutations via `result[key] = value` directly modify the caller's input dict. Demonstration: `d = {'a': 1}; merge_user_prefs(d, {'b': 2}); assert d == {'a': 1, 'b': 2}` — `d` is mutated despite docstring promise.