Post links of the form http://{site}/q/{PostId}/
and http://{site}/a/{PostId}/
do not onebox in chat. I think they should. (Origin of such links: I'm in the habit of deleting UserId from short links, which leaves them ending with a slash as above.)
The slash-less counterparts do create a onebox: http://{site}/q/{PostId}
and http://{site}/a/{PostId}
both work.
And when "q" is expanded to "questions", the trailing slash is no longer an issue: http://{site}/questions/{PostId}/
oneboxes.