What should not be permitted when work is performed on piping that contains or has contained gas?

When work is performed on piping that contains or has contained gas, it is crucial to eliminate any potential ignition sources to ensure safety. Smoking, open flames, and welding are significant risks because they can ignite gas and lead to explosions or fire hazards. This is particularly critical with combustible gases, where even a small spark can result in catastrophic outcomes.

While using electric tools might be permissible if they are rated for such environments and working without proper safety gear is discouraged for worker safety, the presence of open flames and smoking presents an immediate threat during any procedure involving gas. Leaving the area unattended could lead to an uncontrolled situation, but it doesn't pose the same direct ignition risk as the activities mentioned in the correct response. Thus, it is imperative to maintain a no-ignition source policy when working on gas-related piping to prevent accidents.