You are probably mountain flying when some of these things (not an exhaustive list) become significant enough for you to start feeling a bit unsettled if you haven't done it before (or even if you have):
You're moving a lot faster over the ground for a given airspeed than usual (IAS vs. TAS, can catch you out during approaches and turning around in valleys particularly).
The horizon is hidden behind the big hills, so all your usual attitude cues aren't there any more.
You might be flying along beside a ridge and suddenly find you need a heap more power to avoid the ground than normal because you're caught in descending air.
A big invisible hand might reach out, grab the helicopter and shake it around like a toy, then go away (perhaps!).
Your engine acts like it won't pull the skin off a rice pudding.
You're a bit lost, poking around in a valley, fly round a corner and find it's full of cloud. Then you haven't got room to turn around, and you also find that you've been creeping up in altitude and can't hover OGE any more - and the valley floor is covered in trees. Uh oh!
You think 'I'll just duck over that lower part of the ridge line there', fly at it, and find your airspeed is bleeding off because it's a 12000 ft pass, and you've got about 10 degrees nose up to make the picture look right.
You're in a pad with 5 ft on the radalt and 10000 on the pressure altimeter, and little blokes with bows and arrows come out from behind the bushes!
Well enough rambling for the moment...