We have got significant damage on a 206 during auto training on grass. When landed, the aft part of BOTH skids dipped in a sort of potholes, that were left behind by agricultural machinery-tractor, that dug two holes in a distance, exactly matching skids distance. (obviously made by tractor wheels that dug into softened soil during last rain) Potholes were perfectly hidden in grass. When collective was lowered, heli tilted backwards resulting in abrupt stick-forward reaction. A severe mast-bumping followed so mast, M/R Hub, XMSN mounts etc. had to be replaced. You never know what is hiding in the grass as long as it is not verified every time.
The lesson there is survey the ground you are training on before you put it to the test.