In 1.05 Bloody Mary, to quote the Supernatural Wikia
Sam appears to see Jess on the street, in a long white nightgown.
It goes on to say it's unclear as to what he sees, but I wondered if there is any way to find out: What does Sam see?
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Interesting question. I think that the writers of Supernatural intended to do this to create scary feeling about presence of Jess and to prepare the viewers for later Sam's halucinations and other abilities of Azazel's special childrens. This maybe help you: Transcript