Robopon/Nightmare Fuel

Robopon 1

The catridge operates on a battery due to time events and remote control interface. While you can replace it, eventually, the battery will run out, taking your save data with it. The warning message you get upon starting a game with a low battery level doesn't help.
The cartridge periodically lets off a "Ding-dong" when clock-based events occur. This wouldn't be scary except that it happens when it's turned off. When the battery's low, it emits a series of alarm beeps at timed intervals.
Several wild Robopon have too-small CPUs and are prone to violence, confusion, and going out of control. Out of control Robopon randomly selfdestruct and can damage your Robopon in the blast, and this is even worse when you remember they're sentient. A hidden Part called the Spazarm plays into this even more, as it has great stats, but locks your Robopon into OTC mode. You can't call it back or heal it until either it dies or its opponent does.
Miss Amron brainwashes an entire town with her TV broadcasts to love her and do as she says. This even affects people outside the town due to satellite dishes, and many of her followers cna only say her name in adoration.
Illusion Village only appears for a few hours every day. The inhabitants all speak backwards or in roundabout ways, and the village vanishes when its time is up. The village also contains many dungeons full of very strong Robopon, the longest of which has one of the strongest Robopon in the game.
The Serious Business lengths people will go to to hang onto their Legend titles. Kamat used their influence to build an army, and Dr. Disc kidnapped his own daughter and built a giant tower to hide from Cody.
Dr. Zero makes his entrance by killing two of Prince Tail's guards, and his backstory is unusually dark: he had defeated the King and won the Legend1 title, but was attacked and left for dead afterwards, with the King changing the official story so he won. Zero was forced to repair his body with cybernetics, vowing revenge on the Royal Family, and by the end of the game, he completes his revenge and attempts world domination using a castle stockpiled with weapons.

Robopon 2

The Robopon Graveyard that serves as the backdrop to the final battle. You're not told anything about this place or where it is, except that it's where the souls of scrapped Robopon go. The graveyard is full to the point that you're standing on piles and piles of dead Robopon, and Dr. Zero plans to kill you and your team there.
The music in sad situations. At first, it's tear-jerkingly sad, but then the sound effects come in, and it sounds like a computer breaking down.
Cody accidentally helps flood Delica Kingdom with a fishing rod and is sentence to death by hanging. If not for a conveniently-placed time machine, the game would end there.
Dr. Don and Sam continually berate Cody for wrecking their time machines. While funny at first, it eventually drives Don insane.
Dr. Zero's Death Gaia is a floating death machine that completely obliterates Baba Village with a laser and nearly kills Dr. Disc.
There's a bonus roulette effect that instantly kills the opposing team's party. This is accompanied by a creepy, loud sound effect, a huge red skull forming over the battlefield, and the music turning off as each Robopon is scrapped.