There are many mysteries to hagfish goo.
Hagfish are "widely regarded as the most disgusting creature on earth", in part because of how they defend themselves: They excrete a ton of viscous, revolting slime out of 100 glands on all sides of their bodies, blocking the gills of the unlucky predator and essentially drowning them in marine snot. The trick works well enough that hagfish haven't evolved much in 300 million years. Indeed, the slime is so nauseating that there aren't very many predators who'll even dare to attack a hagfish. One of the few? The mako shark.
Still, scientists have long puzzled over one mystery: The hagfish only release the goo after they've been attacked. So how do they survive the first bite? Maybe hagfish skin is so thick the shark teeth don't penetrate?
Only one way to figure it out: Build an artificial mako shark-mouth and have it chomp on a bunch of recently-deceased hagfish, then observe the slime production!
A bunch of scientists actually undertook this awesome steampunk experiment. There's a fun writeup at Popular Science:
To figure this out, biologists at several universities collaborated to make a shark-tooth guillotine, the details of which they published alongside their results in Journal of the Royal Society Interface. Not many animals will even try to attack a hagfish, and mako sharks are some of the proud few, so the researchers gathered mako teeth and glued them to the edge of a metal plate. This plate could be pulled back into the top of the guillotine, compressing a spring as it went, such that it shot down when released. Your bog-standard dropping guillotine wasn’t going to provide enough force to mimic a real shark attack, but the spring-loaded mechanism did the trick. Once they had assembled their death machine, the biologists got a bunch of Pacific and Atlantic hagfish. And since a live hagfish would never consent to sitting still under a teeth-laden guillotine, they were euthanized first. Sorry, hagfish lovers. When the hagfish were unaltered, the shark teeth punctured skin almost every time—but the muscle underneath didn’t get damaged even once. So it seemed like hagfish skin, once thought to be bite-resistant, wasn’t especially tough after all. But skin is much easier to heal than muscle, you can’t expect to come out of an encounter with a shark totally unscathed.
So, tl;dr: It's pretty easy to chomp through hagfish skin, but because it's so loosely attached to the hagfish, it doesn't do much damage – but it does trigger the baneful slime counterattack.