I swear I was not involved in any of the events described below.
In the U.S. Army flight school, Ft. Wolters, TX way back when, solo students were required to get out of the helicopter while it was running, in confined areas generally surrounded by mesquite trees & brush, to survey the area and determine the exit route. We walked around the area around the running TH55, & after pacing the required distance from the edge of the clearing, placed rocks at the point from which we should start. While surveying the area, it was sometimes felt necessary to water the dry brush & grass. One pilot saw the poor vegetation growth, & felt the need to add some fertilizer. He went a few yards away & proceeded to put down fertilizer. Somehow the throttle of the running TH55 wound up to full on, and the helicopter decided to fly. It took off, but didn't fly far before it turned turtle, & pieces flew all over Hell & half of Texas. The fertilizer-applying pilot apparently didn't hear this, or didn't know what was happening, since he still had his helmet on. A dual helicopter passing nearby saw the crash, & landed in the same confined area to see if anyone was still alive. Both the instructor & student got out & ran to the crash site, just before the fertilizee returned. He saw a running TH55, just where it should have been, climbed in, & took off. It took some time before the two unlucky rescuers were discovered missing, & considerable confusion ensued.
Usually, nothing bad will happen when a helicopter is left running unattended. But usually and never are not quite the same thing.