…when one does not actually know.
Have a look at Odd Language #98: Unexpected Answer.
The fireman thought he knew. If the ambulance attendant author had made the same assumption, the girl in the incident probably would have died.
There is knowing, there is not knowing, and then there is the level of hallucination: thinking one knows when one actually does not.
You might consider to checking your assumptions a little more often. I know that I have been caught off-guard because of assumptions that turned out to be wrong.