What is the potential hazard of using a gas cylinder that is not labeled?

Using a gas cylinder that is not labeled poses a significant risk because it can lead to improper usage of the gas contained within the cylinder. Knowing the type of gas is crucial for safety and operational effectiveness. Each gas has specific properties, handling requirements, and potential reactions with other substances. Without proper labeling, personnel may inadvertently use the gas inappropriately, mix it with incompatible materials, or fail to implement the necessary safety precautions. This can result in dangerous situations, including explosions, toxic exposures, or other hazardous events.

In contrast, the other options relate to concerns that are either less direct or not as critical in the context of gas cylinder safety. While leaking gases might occur for various reasons, it is primarily the lack of knowledge about the gas properties that leads to improper usage, which is why the correct answer focuses on that aspect.