There’s something written on this, I believe in the Living the Parshah series. I don’t remember what parshah it’s from. It gives an example of a man who is shopping, and the express checkout line for 10 items or less was empty. The cashier offered to check him out. While he was doing so, another person came with just a few items, and had to wait.
The funny thing was, I had the same thing just a few days later. When someone else came behind me, I turned, apologized, and explained what happened. I avoided even the appearance of doing something wrong.