I had one get airborne during the hydraulic test once, I'm sure it was at full RPM as per the checklist - I must have inadvertently put too much downward pressure on the collective so that the flat steel type lock jumped off the stud and next thing I was in the hover wrestling it around over the pad.
I'm pretty sure from memory I hit the test switch again and then landed normally, with heart rate up quite a bit (this was in a B or BA, can't remember which).
You can hover them hydraulics off with difficulty but it's not something you'd want to do if you could avoid it, particularly if you're not into wind.