Its easily done, as ive discovered on numerous occasions, twice in the Carneddau, once, with interesting results on Cadair Idris, and the other time, i got hopelessly lost in the Black Mountains above Llanthony.
Thankfully they all happened in the heights of Summer, when daylight hours were long, and temperatures forgiving, so there was little panic or worry of darkness descending.
Mist can play havoc with a known route, as the terrain suddenly takes on a different appearance.
The best thing is NOT TO PANIC, and slowly take in your last known position, and try to calculate the time of you last known location.
My last escapade, was going around in circles on Carnedd Llewelyn"s summit for a good hour, trying in vain to locate the path towards Foel Grach.
I knew it was off to my right somewhere, but the navigation even with map & Compass was very difficult.
I even found myself going down hill dangerously towards Yr Elen, i simply had no idea where i was.
My car was in Cwm Eigiau, and after a lengthy merry go round tour of the summit, i had to go to Ogwen Cottage, as it was the only route i could find.
Fun and entertaining in the Summer, but a more serious proposition in the Winter