Yup, this specific situation appears to be a Game Loss. If you change the scenario to an effect that doesn't show it to an opponent, your question is answered by this line from the IPG instead:

If the error is discovered during opening hands, instruct the player to mulligan. Otherwise, do not replace discovered erroneous cards in hands or other sets (such as a group of cards being scried or drawn).

As Andrew and Isaac pointed out, this interesting discussion becomes a rather boring Game Loss - so let's shift this to Regular REL, where it's not a Game Loss - now how would you repair this problem?

d:^D

At regular I would have them desideboard, shuffle while maintaining the position of any cards known in the deck, then reveal another random card off the top. If it was a card thats both in their main and sideboard, things get complicated because they could have scried one at some point. I think I would have them desideboard everything except that card, which could result in them playing a 61+ card deck, but I feel this is cleaner than anything else I can think of.

I understand why this upgrades to a game loss if a player is getting thoughtseized or glimpse the unthinkable, but whats the philosophy behind upgradeing to a game loss if their own effect reveals the card? There seems to be little room for abuse.

Originally posted by Tyrone Phillips:

I understand why this upgrades to a game loss if a player is getting Thoughtseized or Glimpse the Unthinkable, but whats the philosophy behind upgradeing to a game loss if their own effect reveals the card? There seems to be little room for abuse.

I assume you're referring to Comp REL here. It doesn't matter whose effect reveals the card, the potential for abuse is the same: wait to see if the opponent notices anything fishy- if they don't, play on as though nothing is wrong, if they do, immediately call a judge on yourself.

Originally posted by Isaac King:

I assume you're referring to Comp REL here. It doesn't matter whose effect reveals the card, the potential for abuse is the same: wait to see if the opponent notices anything fishy- if they don't, play on as though nothing is wrong, if they do, immediately call a judge on yourself.

Upgrade: If an incorrect card in the deck or hand becomes visible to an opponent (for example due to it being revealed or put into a graveyard) or is discovered by a judge during a deckcheck the penalty is a Game Loss.

I would like to discuss policy behind this. What I believe is true, that we have this upgrade to eliminate situations, when player draws sideboard card, and decide not to act upon this situation until time when opponent cast duress effect (for example) and they says “judge! I think I forgot to desideboard!” (this is event stated in AIPG)

However what we have there is a little bit different. We have situation when player is unaware of the issue until he sees the card and he notifies judge immediately after noticing that there is an error.

Let's consider similar scenarios:
1) I use Evolving Wilds. While searching through my library I call a judge and told him that I noticed that I forgot to desideboard.
2) I activate Azcanta, the Sunken Ruin ability, while looking at top 4 cards I notice an issue and call a judge
3) I resolve trigger from Search for Azcanta, after looking at top card I call a judge

We can all agree, that all those three scenarios do NOT result in Game Loss upgrade.
Then, what if I would resolve Explore trigger with motions similar to Search for Azcanta? (looking at cards myself before revealing it to the opponent for the trigger) and after look, but before reveal, I call a judge to notify him about me failing to desideboard? Incorrect card have not been visible to the opponent, so upgrade does not apply. Then we have two way different rulings depending only on how player is manipulating cards for the Explore effect.

Originally posted by Bartłomiej Wieszok:

Then, what if I would resolve Explore trigger with motions similar to Search for Azcanta? Incorrect card have not been visible to the opponent, so upgrade does not apply.

Doesn't it? Unlike the 3 earlier examples, there's no in-game mechanism to avoid having this card get revealed.

Originally posted by IPG:

Upgrade: If an incorrect card in the deck or hand becomes visible to an opponent (for example due to it being revealed or put into a graveyard) or is discovered by a judge during a deckcheck, the penalty is a Game Loss. This upgrade still applies if the player calls a judge with an effect on the stack or resolving that might cause the card to become visible.

Edited Matt Marheine (May 13, 2019 12:01:12 PM)

Originally posted by Matt Marheine:

Doesn't it?

My mistake there, then indeed in both situations that will now end up with upgrade to GL.

However I stand by my point of that philosophy behind that upgrade is to discourage cheating by not bringing attention to sideboard cards found until it's unavoidable.
Comparison to looking at top card of library from Search for Azcanta is still similar. In both situations player in question want to point out error at the first time they have opportunity to notice it.

So in regards to the original post Q1, after our MSQ this past Saturday we had this exact question come up during debrief. We discussed at length and came to the conclusion that yes the +1/+1 does get added to the creature because of the word Otherwise in the oracle text. Essentially explorer mechanic says hey is that a land? Oh it is, okay put it in your hand. Oh, its something that isn't a land? Give the creature a +1/+1.

In regards to Q2, the second explor, I say yes also because the game wants to resolve as much as possible.

Any input on this? Thank you OP good question!

Edited Randileigh Heckadon (May 20, 2019 06:26:34 PM)

Randileigh - as stated above, at a Competitive REL event, this should be a game loss, so we don't have to worry about a fix.

At Regular REL, I agree with Tyrone here:

Originally posted by Tyrone Phillips:

At regular I would have them desideboard, shuffle while maintaining the position of any cards known in the deck, then reveal another random card off the top.

Originally posted by Brook Gardner-Durbin:

Randileigh - as stated above, at a Competitive REL event, this should be a game loss, so we don't have to worry about a fix.

At Regular REL, I agree with Tyrone here:

Originally posted by Tyrone Phillips:

At regular I would have them desideboard, shuffle while maintaining the position of any cards known in the deck, then reveal another random card off the top.

True sorry I may have misstated. Thank you.