I have a story similar to that.
One time at the gym I saw a woman go up to the hack squat machine, which also has those pins to hold extra plates, and I recall it had 3 45s on either side. Not on the sled mind you...on the pins that hold the extra plates.
So anyway, the sled has no weight on it and she gets on. Gine, the sled itself is a good warm up.
She does ONE REP, gets off and takes a 45 from one side. Right as she does this I think "Whoa! She's moving up fast!"
But she took the plate and out it away on a weight tree a few feet away, as well as one from the other side, gets back on, does one rep, gets off, removes another 45 from either side of the machine, does one more, removes the last two plates, then did about 15 reps.
So she ended up doing 18 reps....one rep each time she thought plates were on the machine's sled, then 15 when she was only doing the sled. Ugh.
Yeah...I watched the whole thing happen. It was like a train wreck; Icouldn't look away.