Just the other day I was in a line at Sears' automotive department. There was a lady in front of me who had brought in a dead battery. She was about to leave

Just the other day I was in a line at Sears' automotive department. There was a lady in front of me who had brought in a dead battery. She was about to leave it with the Sears guy when the man behind me said to her, "It's none of my business, but there are terminals attached to that battery that you might need when you put the new one back in the car" (or something like that). He was right, and they were grateful to him for pointing it out. I thought it was an interesting politeness, actually, that he prefaced his comment as he did. In his place I would probably have started out with "Excuse me, ...", which I don't think is wrong, but his more elaborate intro perhaps made it clearer that he knew he was butting in, albeit with the clear intent to be helpful.