To any remotely safety aware person, a dyno is a terrifying device. This about it: you strap in a 3,000 or so pound machine that is capable to propelling itself to life threatening speeds and then push it to do just that, without moving an inch. The act of running a car on a dyno is, by its very nature, an unnatural and violent act.
For that reason, whenever I see a mobile dyno at a car show or race track, surrounded by throngs of spectators, a bit of a shiver goes down my back. I have seen an engine let go, scattering pieces of now molten metal across the scene, so I stay well away.
If you think I’m being overly cautious, check out this video from 1320 Video of a Ford Mustang being run up on a mobile dyno. The guy in the car runs it hard up through 5th gear, which we can assume is equivalent to about 250 km/h, when the left rear tire literally explodes. The damage done to the car is massive.
Now think about what might have happened had you been standing next to it.
Source: 1320 Video via YouTube