How to get a specific version of a file in Mercurial?

I think you want hg revert -r<rev> <file> (this will change that file to be as it was at the given revision).

As djc said revert alters a file in place to match a prior revision. If you want it not in place you can use hg cat -r revisionid filename (substituting revisionid and filename of course) which will output the file to stdout, suitable for redirecting anyplace you'd like.