My story is a lot like yours except double the fun.
I was at the local YMCA with two rows of treadmills. They were all full with time limits of 30 minutes each and signup sheets.
I'm in the front row and, as time was limited, running relatively fast.
The treadmill opens up next to me and a young male quickly jumps on. He wastes no time and immediately cranks it up to max speed. He is now wildly over striding and his very loud foot strikes resonate throughout the gym.
I'm now fascinated and can't take my eyes off of him when all of a sudden his legs give way and he face plants the deck with a loud thud. In the blink of an eye he is shot off the back and into the treadmill set up directly behind him. His initial scream is followed by moaning. The impact to the treadmill behind him causes that jogger to hit the deck as well with similar results.
Chaos reigns as the loud whir of the empty mills add to the cacophony of yelling and moaning. I jump off my mill, hit his kill switch and help the poor lad to his feet - laughing uncontrollably. The quickly assembled crowds, who are tending to both casualties, are now mad at me because I'm laughing so hard but I couldn’t stop.
The joggers were clearly no worse for wear except for skinned knees and elbows. So there I am trying to explain to the crowd and YMCA staff, through fits of laughter, that it was pretty darn funny but to no avail.
Needless to say, I had to abandon my run and exit stage left as soon as possible to avoid further consternation from the rescue party. The memory of this event still causes me to crack up.