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?

It ends up being not particularly game breaking.

- If you copy Den of the Wolf, you won't get, for example, 3 Dominance phases in a row. Triggering Den of the Wolf repeats the current phase at the end of that phase. So either the original or the copy can create an additional phase, but not both since the second one would no longer be at the end of the "current" phase that it was triggered in. (This is actually the same reason we needed a ruling on multiple Epic Battle cards. I'm not sure we can apply that ruling here.)

- Copying Patchface in a Joust game would have a similar effect. The original and copied "must remain revealed" effects run parallel and have the same duration, so copying doesn't gain you anything. In a Melee game, you could choose another opponent, but that would be no different than two different players using their own copies of Patchface in the same round. Pretty much the same thing with Tyrion's Chain in Melee. They could make one round very one-sided, but in melee that's not particularly game breaking (or uncommon - careful selection of Titles can do it, too).

Moral of the story, BTW, is that yes, they can be copied. Copies of "once per game" effects do not exceed the limit any more than copies of "once per phase" effects exceed their limit - reason being that you are not triggering the "once per game" effect a second time.