At the hospital where I did my internship, we used to get a lot of patients from prison. The main thing about prisoners is that you can never, ever ask them why they're in jail. You can't know and you probably don't want to know. Also, it's probably drugs.
When I was working in the ED, I saw a male patient with what looked like an abscess on his leg. One common cause of abscesses at the hospital where I worked was "skin popping." This means injecting drugs like meth under the skin instead of in the vein. We used to see this, like, a couple of times a week.
So of course, I quickly jumped to the conclusion that this jailbird patient had been skin popping at some point and now had an abscess from it.
Me: "Do you use drugs?"
Me: "So you've never injected any drugs under your skin?"
Patient: "No, I don't do drugs."
Me: [very skeptical]
Police officer: "It's true. His problem is credit cards."