Why can a stonefish sting be dangerous?

A stonefish sting is dangerous primarily because these fish possess a potent venom that they can inject through grooves in their dorsal fin spines when stepped on.12

Because stonefish are sluggish, bottom-dwelling creatures that blend almost perfectly with their surroundings—such as rocks, coral, or mudflats—they are extremely difficult to see, which increases the likelihood of accidental contact. The wounds caused by this venom are known to be intensely painful and can, in some cases, be fatal. These fish belong to the order Scorpaeniformes, which also includes various scorpionfish that share similar venomous characteristics.12