This is something I was starting to think about, and mostly applies only to Legacy (and other non-sanctioned formats):
Do card effects that are "Limit once per game," such as Den of the Wolf or Patchface or Tyrion's Chain able to be copied? I can't find a direct ruling in the FAQ, but it seems to me that the limitation would imply that copies would exceed the once limit, and thus no be allowed (unless the original effect were canceled perhaps?). If this is not the case, would this be an issue that should be addressed in a future FAQ, or is this something that should be allowed and is not particularly game breaking?