|Even NP Harris does it...|
Well, eavesdropping and then joining in the conversation, without the other person/people being aware.
I have done this with pissed-up clubbers queuing for a kebab, old dears as they wait for their pension in a post office queue, chavs hanging around in gangs in shopping centres and various random people talking loudly on their mobile phones. Obviously, some of these situations are more dangerous than others - I leave it to you to decide which.
Julie is somewhat unsurprisingly disapproving of this habit of mine, possibly due to the potential for fist & face meetings. In my defence, I do have to call hypocrisy on this one, as Julie does engage in this on occasion. Of course, when I call her out on it, her reply is normally, "...that's different."
Joe Jackson had it right.
One time, we were in our local shopping centre, thinking about having a cuppa and perhaps a bun. Ahead of us in the queue were a couple of elderly women who were definitely in the market for some baked goods. I had already been feeling mischievous, and when one of the women asked her companion a poorly-phrased question, my reply was more of a knee-jerk reaction than anything that was thought through...
...here comes Muffin, Muffin, the mule....