What surface would be unsafe to use a carpenter's hammer on?

Using a carpenter's hammer on hardened steel would be unsafe due to the potential for damage to the hammer. Carpenter's hammers are designed primarily for striking wood, which allows for a certain level of force without causing significant wear or damage to either the hammer or the workpiece. Hardened steel, however, is much denser and tougher than wood, and striking it can result in chipping or breaking the head of the hammer.

Additionally, the metal surfaces on hardened steel can lead to a risk of glancing blows that can cause the hammer to slip, posing a safety hazard both to the user and the surrounding area. It is generally advisable to use a hammer specifically designed for metalworking, such as a ball-peen hammer, which is built to withstand the impacts involved in working with tough materials like hardened steel. Therefore, using a carpenter's hammer on such a hard surface is not suitable for safety and equipment preservation reasons.