Most shops have a public area (waiting room) and a client area. If you're not in the chair, you don't belong in the client area, unless the artist says okay. One shop I know, you have to be buzzed into the client area.
I've heard of artists being beaten up (goons sent from a competing shop), and an attempted armed robbery. I won't mention the shop, but the artist raked the tattoo gun across the bad guy's face and he ran. I have given thought to the amount of $$ I may have in my pocket when I sit in the chair. Late in the evening, the artists may have some serious cash on them.