Walking with my wife the other day, I turned around and realized that the person behind me was, in fact, someone else, and my wife had stopped to look in a shop. I said to her
1a Oh, I thought that guy behind me was you.
Possibly because I had a bit of cognitive interference with "Oh, I thought you were behind me", I was tempted to say
1b Oh, I thought you were that guy behind me.
But on reflection I realize that 1 sounds right (felicitous?), and 2 sounds wrong (infelicitous).
Edit I've used these terms from now on at P Elliot's suggestion
As another example
2a The boy thought the woman whose hand he was holding was his mother.
Is felicitous, while
2b The boy thought his mother was the woman whose hand he was holding.
3a I thought that log was a bear.
3b I thought that bear was a log.
Both are felicitous, although they point to different meanings.
My thoughts so far: definiteness might be important – playing around with how things sound, "a guy" vs. "the guy" vs. "that guy" vs. "something" seems to shift things a bit.
Alternately, maybe it has something to do with "was you" being qualitative rather than existential – that is, the meaning is not A=B but rather something closer to "A is B-like".
Or, perhaps, some basic and fundamental aspect of semantics (/pragmatics?) that I'm forgetting/never learned.