The use of a cold chisel with a mushroomed head is:

The use of a cold chisel with a mushroomed head is considered bad primarily because there is a risk of flying chips from the head during use. When the head of a chisel becomes mushroomed, it has deformed due to repeated hammering, causing the edges to raise and become uneven. This uneven surface can lead to two issues: first, a compromised striking area, which can cause uncontrolled movement of the chisel when struck, and second, increased hazards due to debris being dislodged, which can pose safety risks to the user and others nearby. The integrity and safety of tools are critical in any work environment, and a mushroomed head compromises these factors significantly.

Utilizing a chisel in such a condition not only proves ineffective but also endangers the operator, making it essential to replace or properly maintain tools to ensure optimal performance and safety.