Teaching a Rescue Diver class two years ago, I made a point of surreptitiously briefing all the divers I could see around / near the quarry entrance we were using, before we started a Distressed Diver at the Surface exercise immediately after lunch. I had my DM slip away as the students were finishing their lunch, so he was positioned about 80 yards from shore when the students came down to the water. As soon as he saw them, he went into his 'act' - yelling for help, flailing about, splashing, etc. I pointed out to the students that there appeared to be a distressed diver in the water and they need to do something, quickly. Turns out, a couple of divers had come into the quarry area during our lunch break, and were parked (and setting up) out of sight around a bend in the shoreline, on our left. So, I hadn't seen them or alerted them to what was happening (nor could they hear me call out, 'This is just a drill'). As my students were (somewhat slowly, I thought) getting in the water, and the first swimmer was heading toward the 'distressed' DM, I saw a diver (one of the new arrivals) come racing across a dock about 50 yards from us on the left, fins and mask in hand, make a tremendous (and graceful) leap into the water, and begin swimming vigorously toward the DM. He reached the DM well before the students, and was not at all pleased (a euphemism for his actual reaction) when he found out he had responded to an exercise, not a 'real' distressed diver'.
Lots of apologies were made afterward (by my DM, by me, by both of us again, etc.), and lots of compliments were paid to (and finally accepted by) the unaware diver regarding what was an almost picture perfect response to a Distressed Diver at the Surface. I also used the experience as a teaching opportunity, to point out how quickly my students should have reacted, in contrast to how slowly they actually did react. (But, was I ever embarrassed.)