Some years ago I had a brief consulting gig. I showed up in my standard t-shirt and jeans. There was another consultant there, whom I hadn't previously met, in a suit. When he sat down at the computer, I had to stifle an impulse to offer him assistance. Then I had to laugh at myself.
Since then I have tried to be more aware of my assumptions and less swayed by appearances, but I haven't always succeeded. Heh, I recall another incident where I roundly ignored an attractive woman at an entrepreneurship meeting, only to find she was a CEO and one of the panel speakers.