Things can get dangerous
According to Real Food/Fake Food, "Consumers ordering white tuna get a completely different animal, no kind of tuna at all, 94 percent of the time."
Not only that, but the fish they substitute for tuna, escolar, is commonly referred to as "Ex-Lax fish" in the seafood industry, for what should be obvious reasons. If you are wondering why serving this diarrhea time-bomb is still legal... well, it isn't in many countries (Japan notably one of them) and it was banned in America by the FDA in the early 1990s, only to be unbanned in 1998.
"So when people think they are sick because their tuna has gone bad, it's way more likely they never even had tuna in the first place," Olmsted said. Though escolar-for-tuna might be the most widespread and particularly harmful instance, tilefish (on the FDA's do-not-eat list for children and pregnant women) is often swapped for red snapper or halibut, and tilapia (which is admittedly less harmful, but still shitty) is swapped for tuna, too.