First, I was carrying my items -- wallet, phone (iPhone, pretty non-descript, black case, nothing colorful or shiny that would attract goofy humans), keys, and the two things I was buying.
I went through the usual bit at the register, and I sat my phone and keys down to get my plastic out of the wallet to pay.
The cashier, a young male, picked up my phone, pressed the button and pulled up the wallpaper that comes on. Then he starts asking me about who the picture is, etc... I'm not real talkative, but I did get out "what the fuck are you doing?"
That got the phone returned to the little shelf by the credit card machine. I finished the transaction and told a person from the cash office what happened (she was doing pickups two registers away). I got the usual "I'll see that someone in management hears about it, he was wrong to have done that" line.