I've done it a few times, both with leftovers like you've got, or with a new keg like Drew mentioned. No problems at all.
I would have stopped by, but it's nearly 3000 miles
I would have stopped by too (I'm less than 15 miles away)...but found this thread too late!
I've transferred leftovers from commercial kegs as well with no problems. Usually though, if is something bland like Coors or Bud, I'll dump a couple bottles of Guinness or something into it to give it a bit more flavor. Works surprisingly well, too.