The accident scenario with welders usually involves confined spaces and an acetylene torch. Due to the confined space the core of the flame is dipped into the molten material. This will result in a small explosion wich sends a shower of big hot sparks into the vicinty. Again due to the confined space some will always find a way to enter the pockets. Those sparks have enough heat and energy to melt through the tank of a disposable lighter OR ignite the butane gas. After the first hit the gas leaks into the clothing and can be ignited there. With the proper timing this causes a small fire. The resulting injuries are first degree burns or if you´re really unlucky second degree burns. In the vicinity of the pant pockets this would be bad anyway.
The manufacturers warn that the disposable lighter should not be kept at temperatures above 60°C. That temperature is easily reached in a parked car on a hot summer day. So it´s no surprise that leaving it there is an accident waiting to happen.