1) Robinson says it's 80% RPM, +1% per 1,000ft DA (so 85% at 5,000ft DA). They won't say if that number includes any safety margin, whether it is in or out of ground effect, in forward flight, climb, descent or a hover. I suggest not to even think about trying it.
2) The blades may fold up, and the retreating blade can flap down hard enough to cut the tailboom followed by the upper half of the cabin, which contains the unfortunate occupants' heads.
Both has been recorded on video. The tell-tale signs, such as the cut tailboom behind the strobelight, and blades bent upward a couple of feet from the hub, can be seen on many Robinson wrecks.