708.9. If a face-down permanent or a face-down component of a merged permanent moves from the battlefield to any other zone, its owner must reveal it to all players as they move it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all players as they move it. If a player leaves the game, all face-down permanents, face-down components of merged permanents, and face-down spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents, face-down components of merged permanents, and face-down spells must be revealed to all players.

Is there a penalty for failing to reveal facedown permanents / facedown spells at the conclusion of a game/match? If so, what infraction has been committed? 

Is there a difference for why the permanent is face down (cast for its morph or disguise cost vs manifested/manifested dread)? Does this extend to cards face down in exile due to Foretell? 

Take a look at the Penalty Guide, 2.3, Hidden Card Error - especially the last part that calls out things like Morph or Disguise.

Note also that it specifies face-down permanents - cards exiled by something like Foretell do not have to be revealed.

d:^D

Hi Scott I have infact read the IPG and even deep dived back to previous versions to see where Morph was moved from GRV to HCE however the only mention of face down cards in the current IPG is the following 

“Upgrade: If a face-down card cast using a morph or disguise ability is on the battlefield during the game and does not have the relevant ability printed on the card, the penalty is a Game Loss. If the player has one or more cards with the same ability in hand, has not added previously unknown cards to their hand since casting the card found in violation, and has self-reported the error before an effect that would reveal it is played, the upgrade does not apply and they may swap the card for a card with the appropriate ability in hand.”

Which does not (in my mind) cover failure to reveal cards at end of round unless we’re determining that “If the set of cards that contained the problem no longer exists, there is no remedy to be applied.” applies since we have cards that should have been revealed but weren’t and a game that has ended. 

additionally Foretell also has to be revealed so I fail to see why it is exempt here. 

702.143f If a player leaves the game, all face-down foretold cards that player owns must be revealed to all players. At the end of each game, all face-down foretold cards must be revealed to all players.

Edited Rhianna Milton (Yesterday 12:31:12 AM)

The breaking the rule you stated, i.e not revealing face down stuff, Is a Game Play Error.
Now normally, any rule breaking not covered in the other sections would be a GRV.
And the morph reveal used to be under GRV a long time ago.

Then came Hidden Card Error.  HCE is a bit broader than its predecessor, "Drawing Extra Cards" and it absorbed this aspect of GRV.
 

Not revealing the face down cards and losing their identity falls under the first sentence of HCE.
"A player commits an error in the game that cannot be corrected by only publicly available
information. " 
Once the cards are smashed in with other cards, we can no longer correct that error with only publicly available information.
and the CR supports this by saying the cards need to be revealed at the end of game.  The CR cant really direct you to do something outside of the game, now can it.

This rule applies to all face down permanents and spells, regardless of how they became face down.
A separate rule applies to foretold cards, which you quoted.

It helps to think of GRV as "GRV - Public" and HCE as "GRV - Private", in this case.  Its not a 100% mapping, but its 99% 

The correct infraction is HCE.  The penalty is Warning.  The fix is: "wag your finger at them".

The upgrade does NOT apply if you fail to reveal at the end of game.  It used to apply, but the wording changed many years ago to exclude it.
Basically players were distracting their opponents at the end of game and calling a judge for free wins when the error happened.  And then we realized a few things...If you got time to wait to pounce on your opponent for not revealing, you have time to say "hey, lemme see that card".  Games dont just end because one player says so, it takes two to acknowledge a loss.  So if you really wanna know what that card is just scoop at the same time you say "hey, lemme see that card".  Since the opponent has the opportunity to play a role in the reveal, an upgrade is not appropriate, and the upgrade wording was changed to reflect that.

 

Scotts reply was a little brief, and probably assumed that pointing out the HCE upgrade mentioned morph was sufficient for getting you into the right infraction.

For the CR, and why Foretell has a separate rule for its reveal instead of being lumped in with the other face down stuff, is most likely due to the fact that things can end up face down in exile for a ton of different reasons, so having a generic "face down cards in exile" rule doesnt make sense.

I will acknowledge a "quick answer" mode (heh); also, I had the Foretell mechanic confused with something else.

I'm not actually sure about Foretell; it seems that the same philosophy as Morph would apply - i.e., you have to reveal to verify the legality of the play.  Specifically, that a Morph has a Morph cost, or a foretold card has a Foretell cost (unless foretold by something like Dream Devourer.

 

d:^D

Thank you Bryan for taking the time to give a thorough explanation including a break down of why the game loss upgrade for this was done away with. Crystal clear now, appreciate it 

>I'm not actually sure about Foretell; it seems that the same philosophy as Morph would apply - i.e., you have to reveal to verify the legality of the play.  Specifically, that a Morph has a Morph cost, or a foretold card has a Foretell cost (unless foretold by something like Dream Devourer.

 

Foretell is somewhat distinct from Morph in that a face-down card in exile isn't doing anything, while a face-down card on the battlefield can attack and block. If you accidentally Foretell a non-Foretell card, the only effect you've had on the game is psychological (that is, you made your opponent play around your Foretold card). But if you accidentally Morph a non-Morph card, you might have dealt damage, deterred attacks, and otherwise more viscerally affected the game.

 

That's not to say that there's no disruption from a mis-Foretold card, just that it's generally not as disruptive as a mis-Morph, so it doesn't to be penalized as harshly (in addition to what bprill said about how easy it is to get other cards face-down in exile, most of which don't need to be revealed to prove their legality).