Which of the following defects can occur as a result of improper soldering technique?

The identification of "cold solder joint" as a defect resulting from improper soldering technique highlights a critical aspect of soldering quality. A cold solder joint occurs when the solder does not properly melt or flow around the connection, often due to insufficient heat being applied during the soldering process. This improper connection can lead to poor electrical conductivity, which may result in intermittent failures in electronic circuits. A cold solder joint appears dull or grainy, contrasting with a smooth, shiny finish of a properly soldered joint.

In terms of other defects mentioned in the choices, while they may relate to electrical performance, they do not specifically stem from soldering technique. Electrical component overload can arise from design issues or component ratings being exceeded rather than from soldering practices. Insufficient voltage supply is typically a result of circuit design or power source issues, and thermal expansion errors are commonly related to material properties and environmental factors rather than the soldering process itself. Therefore, the nature of a cold solder joint directly reflects the consequences of poor soldering technique, making it the most correct choice in relation to the question posed.