I had always been told I was intelligent and clever. When it came to the exams everyone takes as a 15/16 year old in the UK I distinctly remember thinking to myself something like: "everyone has to be able to pass these exams, so I should be able to pass easily without much effort since I am one of the clever kids".
I didn't cheat, but my line of thought led me to a fairly disappointing set of results (as you'd expect).
Even now I think of myself as "smarter than the average bear", but I have the feeling that people were just telling me that because they were my parent/grand parent/friendly neighbor/aunt etc and now it is SET IN STONE IN MY BRAIN after years of reinforcement as a child.
What really confuses me though, is how can I think I am smarter than average, yet suffer from near constant impostor-syndrome? :-)