Fake Client Con
This is a risk not to clients, but to practitioners. A fake client con job is designed to extort money from studios, usually by a traveling pair ("mother" and "daughter") who will hit all the tattoo studios in a town and then move on.
The con goes something like this (there are a ton of variations, all designed around local laws):
A young looking girl (16-18) comes in with her "mother" and wants to get a tattoo. Usually the girl will already have tattoos. If the area allows minors to be tattooed with parental consent, they'll present her as underage but with a parent (along with supporting ID). If it doesn't, she'll either give ID or have the "parent" claim she's of age and give consent. They will try to do all of this with as little paperwork as possible, and skate just outside the law (because they know the studio is more likely to bend the rules if it's "just a little" and "she already has tattoos," after all). The day after the studio does the tattoo, the studio will get a call from "the real mother" informing them that they've just tattooed a minor, and that the "parent" was just a friend (or aunt) who went along without permission. The parent will offer to settle it on the spot if the studio hands over $1,000 "to pay for tattoo removal." Usually the studio will simply pay to avoid the huge potential headache.
Studios can defend themselves by knowing their local laws and sticking to them. Don't make exceptions; remember that even the people who appear trustworthy are still just total strangers who walked in off the street. If the law requires a certain type of ID, get that ID from all required parties and DOCUMENT DOING IT. If documentation of proof of ID is obtained, the scammers have little opportunity to do any damage legally (although they could generate some very bad press).
As far as performing any piercings or tattoos without identification is concerned, remember one important thing: when someone asks an artist to do this, they are asking him or her to break the law in a way that risks the artist's livelihood!