Why should you always disconnect at least one lead of the thermostat?

Disconnecting at least one lead of the thermostat is important to prevent damage to the heat anticipator. The heat anticipator is a component within the thermostat that helps regulate the heating system's operation by preventing the system from overshooting the desired temperature. When working on the thermostat or the heating system, if you do not disconnect a lead, the thermostat may send power to the heat anticipator and the heating system while you are attempting repairs or replacements. This can lead to excessive current flowing through the anticipator, causing it to overheat and potentially fail. Therefore, disconnecting a lead ensures that the heat anticipator remains protected, thereby maintaining the integrity and proper functioning of the thermostat and the overall heating system.