I used to work for a grocery in the IT dept: repairing POS registers, printers, etc. I don't dress like the staff as they have uniforms.
I was elbow deep in fixing a register at a lane with parts and tools lying around on the counter. I'm crouched down, squinting and scowling at the stubborn beast before me. Why is it doing this thing when I put the other thing in its place? Okay, jiggle this here, connect that wire there...
Suddenly I hear a noise and look up to see an old man pulling up his cart. He starts unloading items onto the belt.
Me: "Sir, this register is closed."
Old man: "Then open it and ring me up! The other lanes have long lines and you need to assist your customers!"
Me: "I am not an employee of this store, sir. I'm repairing a broken register."
Old man: [he has been completely unloading his cart as we converse and he punks his last item on the belt angrily] "You don't have a line, and you need to do your damn job and ring me up!"
He stood in front of the payment terminal, glaring, waiting for me to start ringing up the items.
Y'know what? I don't have to kiss YOUR ass sir, so you're about to get a wake up call. With a grim smile, I kept working on my problem. Five minutes (and several customers through other lanes later) I finally finished.
I packed up my tools, cleaned up the area, did some quick testing. It's working again! It's ready to go, all it needs is a manager to log it on and take a drawer!
The old man still stood there, staring at me.
I picked up my tool bag, smiled sweetly, and told him, "An employee from this store can help you if you go to a lane that's actually open, sir," and walked away.
I did let a manager know about the old man and our conversation. The manager just sighed and said that he'd take care of it. As I walked out the door, the manager was talking to the angrily gesturing old man. By the manager's cross-armed posture, bullshit was not being tolerated.