Does display-buffer display the buffer or not?

[Lennart Borgman]

The doc string for displayb buffer begins like this:

  Make buffer BUFFER-OR-NAME appear in some window but don't select it.
  BUFFER-OR-NAME must be a buffer or the name of an existing
  buffer.  Return the window chosen to display BUFFER-OR-NAME or
  nil if no such window is found.

I do not understand the last sentence. Could it happen that the buffer
is not displayed? In that case, should not that be an error?