to think first impression is normally correct...

[ladystarkers]

After you have spoken to someone?

It seems to me I’m usually right about people even though I might have ignored my gut instinct —and massivley regret it—

First impressions are usually right.
But you have to take into consideration your own state of mind.
If your mood is annoyed or angry for some reason you are less likely to give much time or effort in getting on with the new person or vice versa.

Like other posters above I’d trust a creepy vibe instantly - have never been wrong about this. I also think I’m personally good at setting aside racial stereotypes etc and getting to know a person quite quickly. So I can imagine if you’re like me that a first impression is more than sufficient to make your mind up about someone. But many people mistake extrovertedness for friendliness etc and so first impressions alone put shyer folk at a disadvantage.

Actually personally I quite like people who stuck to first impressions, because the first one I make is usually good! I'm chatty and friendly and do initial small talk really well. It's only after that that my social insecurity kicks in and I probably seem more reserved on subsequent meetings, when I start worrying more, especially if I really like the person, that they are going to decide that I'm not worth knowing. I have a zillion good acquaintances, but very close friends, far fewer!