I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be"

No-one seems to be questioning this, but it conflicts with my experience as I often overestimate people's intelligence. This may be a failure mode confined to me, but I often meet and talk with people, who I'll "code" as being quite intelligent because they're well-spoken and broadly knowledgeable, (and tall and attractive and popular and hello-there-Halo-Effect). I'll then have to reassess this at a later point, generally by seeing them "in action" trying to reason about something.