I think i saw someone who wasn't really there, it's freaking me out. am i going crazy?

That's not enough: For anyone to think about your going crazy. I can believe that it freaked you out and depending on who it is or was and the whole story, it can cause intense feelings too. I have no way of knowing how it happened, but visual hallucinations of a particular person are not the way symptoms of psychotic conditions usually begin in an otherwise well adjusted (or even average) adult.

Not necessarily: If you're having visual hallucinations, you need to see your physician because there's a long list of possible medical (not necessarily psychiatric or "going crazy") causes -- including drugs & alcohol. Certainly if this continues, these need to be investigated. I don't know the circumstances of what you saw, but severe mourning can also be accompanied by visions of the person who has died.

"Going Bonkers"?: For a fascinating ted talk by neurologist dr. Oliver sacks on hallucinations & what they can tell us about our brain see http://bit.Ly/10d8ws7. Hallucinations don't need to mean you are "going bonkers, " a phrase sacks likes to use.