I went into my bank once, filled out a deposit slip and handed it and my check over to the teller. She looked at it and asked if I'd written anything on the back. No. She showed me that someone had written upon it, "This is a stickup." She had me fill out a new slip and placed my original in an envelope for the police. I never heard if they found out who'd done that.
You know, that's a very smart play on her part.
If you HAD written it on the back, then you would have said YES, and she would have treated it as a robbery.
Since you said NO, she realized that you picked up a publicly available document , and weren't a real robber.
She could have just hit the alarm button, and caused you no end of grief.
Now, writing "This is a stickup" on every deposit slip (or printing it on someone's order of deposit slips) WOULD be quite a practical joke!