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Nightmare Fuel: Robopon
The first game 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 message you get upon starting a game with a low battery level doesn't help.
The first game 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—again, while turned off.
In the first game, several wild Robopon have too-small CPUs and are prone to violence, confusion, and going out of control. Out of control Robopon randomly selfdestruct. This is even worse when you remember they're sentient.
There is a hidden Part called the Spazarm that has great stats, but locks your Robopon into OTC mode. You cannot call it back or heal it until either it dies or its opponent does.
Miss Amron brainwashes an entire town with her TV broadcasts; this even affects people outside the town due to satellite dishes.
Illusion Village, in the first game, only appears for a few hours every day. The inhabitants all speak backwards or in roundabout ways. And what would happen if you were in it when it vanished?
The village also contains many dungeons, the longest of which has one of the strongest Robopon in the game.
The lengths people will go to to hang onto their Legend titles. Kamat used his/her influence to build an army, and Dr. Disc kidnapped his own daughter and built a giant tower to hide from Cody.
Dr. Zero's backstory. He had defeated the King and won the Legend1 title, but was attacked and left for dead. Zero was forced to repair his body with cybernetics, vowing revenge on the Royal Family.
By the end of the first game, Dr. Zero has completed his revenge on Prince Tail, imprisoned Princess Darcy, and controls everything on Porombo Island. Cody is the only one that's able to stop him at all.
In the second game, the Robopon Graveyard. 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 dead Robopon, and Dr. Zero plans to kill you and your team there.
Zero: You'll regret this! We'll make no distinction between you and your Robopon!
Also in the second game, the music in sad situations. At first it's tear-jerkingly sad, but then the sound effects come in.
Cody accidentally helps flood Delica Kingdom with a fishing rod. The punishment? Death by hanging. If not for a conveniently-placed time machine, the game would end there.
At the end of the second game, it's revealed that Cody defeating the villains caused their actions in both games thanks to time travel.
In the second game, Dr. Zero's Death Gaia completely obliterates Baba Village and almost 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.