I used to get cannabis 'flashbacks': I'd be walking along, or doing an every day thing, and suddenly I'd be very stoned for about 5 - 10 seconds, then I'd go back to normal.

They definitely weren't natural highs, though: the highs would feel exactly the same as being really blazed on weed; plus I'd never experienced it before smoking cannabis, and I rarely experience it now (mind you, I always seem to have weed, so that explains why).