A 45-year-old man returns 1 week after a large MOD composite was placed in 25. He has sharp pain when he bites on hard food and on release, but no spontaneous pain, and the tooth responds normally to cold. Articulating paper shows no premature contact. The notes record that the composite was placed in one 5 mm increment of a conventional (non-bulk-fill) resin and cured from the occlusal only. What is the most likely cause?