Sometimes, you can get pretty much everything you need to know from the title of an article alone. To whit:
First documented attack on a live human by a cookiecutter shark (Squaliformes, Dalatiidae: Isistius sp.)The article itself is in a journal I don't have access to, but I can read the abstract: the person attacked was a long-distance swimmer in Hawaii and was bitten twice. The bite was treated with skin grafts, but still took nine months to finish healing.
Cookiecutter sharks are one of the more fascinatingly evil fish out there. They are small, as sharks go (up to about 50 cm, tops) but have proportionately oversized teeth that are arranged in a tight, single-row array that can be protruded outwards to take a neat plug out of the flesh of a larger animal: hence the name of 'cookiecutter'. The effectiveness of the cutting tooth row is maintained by being replaced all at once, rather than individual teeth being replaced piecemeal as in other sharks. Cookiecutters are rarely encountered by humans as they are generally deep sea fish, living below the light zone, but like many mesopelagic animals they appear to migrate closer to the surface at night (Papastamatiou et al. 2010). Bioluminescent photophores behind the head have been suggested to function as a lure, drawing larger fish, dolphins, etc. into range of an ambush. Cookiecutters have very catholic tastes, and evidence of bites has been recorded from just about any decent-sized pelagic animal. They will even bite the external insulation on submarines.
Given their lack of pickiness, it is hardly surprising that a cookiecutter would take a bite out of a human. Of course, humans very rarely venture into the pelagic environment in which cookiecutters can be found. The very fact that the Hawaii indicent is the first confirmed attack indicates how extremely rare this would be expected to be. The Wikipedia page on cookiecutters refers to possible attacks on shipwreck survivors (though the source page linked to does not provide citations for such reports), and the body of a drowned fisherman was recovered in Hawaii with cookiecutter bites. But unless you happen to be swimming in the open ocean at night, your chances of being bitten by a cookiecutter are low.
Papastamatiou, Y. P., B. M. Wetherbee, J. O’Sullivan, G. D. Goodmanlowe & C. G. Lowe. 2010. Foraging ecology of cookiecutter sharks (Isistius brasiliensis) on pelagic fishes in Hawaii, inferred from prey bite wounds. Environmental Biology of Fishes 88 (4): 361-368.