How do I enter a multi-line footnote in LaTeX?

My intuition was

Lorem ipsum\footnote{long footnote
  that spans a whole
  bunch of
  lines.
}

But regardless of where I put the { and } in relation to the footnote text, I get the following error:

Latex Error: ./mydoc.tex:142 Package inputenc Error: Unicode char \u8:― not set up for use with LaTeX.

Solution

Footnotes are filled in the available space just like any other paragraph: you just write

\footnote{ 
    a whole lot of text that goes on and on and on and...
    ...
    and may consists of multiple sentences. But after a while
    ... 
    it finally comes to a stop.
}

That is not your problem. Looking at the error message (which I don't recognize from personal experience), I'd say your problem is character set or font related.

Is you editor using unicode?