It's the final round of a Modern PPTQ. Adrian is playing against Noel. On the first turn of game 2, Adrian plays a Scalding Tarn and fetches a Steam Vents to cast Serum Visions. After Serum Visions resolves, the players notice that a Deserted Lighthouse stuck behind the Steam Vents and is on the battlefield and they call for a judge. What do you do? (Assume you can rule out cheating.)

For bonus points, what if the extra card was not a land but something else, such as a creature, or an instant?

I won't ask Toby to write this in policy, so we'll just have to settle for ‘O’pinion: shuffle the card that's supposed to still be in the library, into the library. Call it L@EC if you feel a need, but this is a simple, common-sense solution, IMO.

d:^D

I think a GRV, deviate to shuffle the extra land back in is a reasonable fix. What's the reasoning behind the L@EC penalty? Also, what if this was discovered much later in the game?

Well, if the players “noticed” a Deserted Lighthouse, (or other card) it stands to reason they must have looked at it. And if it is supposed to be in the library, it stands to reason that they weren't likely entitled to do so.

Therefore L@EC seems to apply.

But L@EC also says this:

Once a card has been placed into his or her hand or if a player takes a game
action after removing the card from the library, the offense is no longer Looking at Extra Cards.

And a Serum Visions has been resolved.

If cheating has been ruled out, we can look at this as GRV, incorrect resolution of fetch, give that penalty. L@EC hasn't occurred as the player was allowed to see his library. I'd shuffle the card back in, preserving the previous scry, and give a warning. I feel this is the closest to policy, though a minor deviation.

Something very similar happened to me at a PTQ a couple of years ago. I was a FJ, and L1 at the time. A player discovered a card in his hand had another card stuck to the back of it. I was the judge called, and I immediately passed it on to the HJ, as I didn't have a clue about how to resolve it! The HJ had a bit of a think, and determined DEC, and a GL.