I think the most common insult to my intelligence is when people assume that I don't know about things because of my age. In my life, I didn't have a lot of friends growing up, so I found myself hanging around with the adults at most functions. My father was in the Air Force, so we moved around a lot and I had the tendency to watch people and learn what they were doing. By the age of 18, I had already worked for many "Handymen" who taught me much about their individual trades. By the age of 21, I had already lived in more places than I could count and held nearly every job under the sun that didn't require a specialized degree and even some that did.
So, while working somewhere and some troubleshooting was needed, I comfortably would give my input. The problem was when other people would look at me and ignore what I said because I was "just a kid" and do something else that would end up failing, being more difficult, more expensive, or a combination of problems. What really would pour the salt in the wound would be that after their plan failed, someone else would propose my idea and it then would be embraced and applauded.
Finally I moved to a place and got a job where my intelligence, IQ (148), and thought processes, although unorthodoxed, are appreciated. The Corporation that I work for now has my handywork all over the place. Improvements on machines have been built, arrangements have been made for better efficiency, and even some of my own polocies have been indoctrinated into the company handbook.
So, although it has been a long and trying time, I finally recieved the recognition and respect that I felt I was due. It was nothing over the top either. It all came down to "Good job", "I never thought of that, that's a good idea", or even more simply "Thank You".