Recently a friend had an operation in which she was given medication to make her forget the operation (it was an eye operation done under local anaesthetic, and apparently the "scalpel coming at your eye" memory causes nightmare reactions).
So, she must have had an instant of terror on seeing the scalpel cutting into her eye, but now has no recall.
If so... was she ever terrified? If there is no memory of it whatsoever, can we call it terror? If so, how do any of us know that we haven't been similarly terrified?