You're also liable to perceive people with low social skills as less intelligent than they are
Also, you are likely to underestimate the intelligence of people who are not native speakers of your language (because language skills influence all data your heuristics get), or have some speech problems.
Is this based on theoretical reasoning or do you have anecdotal evidence for it? I'm genuinely curious because I don't know what to think on this issue.
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.