Tactical Suicide Boss: Mr. Carpainter will continue to shoot lighting at you even after the Franklin badge deflects it back at him for massive damage.
Tag Line: The Japanese tagline stated this was a game that adults, children and ladies could enjoy. The American ads instead warned that the game stinks.
Terminator Twosome: Giygas apparently destroys the universe, forcing Buzz Buzz to go back in time to find someone who can stop him, and Starman Junior goes back in time to stop Buzz Buzz. Starman Junior fails although Buzz Buzz ends up dying anyway.
The towns are named by numbers, for their climate, or for musical references.
Also, in the Japanese version, the "Don't Care" preset names for the main characters were grouped by the following themes: The Beatles, Super Mario Bros., primates, the Japanese band SMAP, the character's signature head ornament, and dog commands, in addition the starting "Ness, Paula, Jeff, etc." Of course, those sets, like some other things, got Lost in Translation.
Parodied with Slot Machine Brothers — Pincho, Pancho, and Tomás Jefferson.
Some fans have noticed the following in the town names: Onett + Twoson + Threed + FoursideSummers/= Tenda Village. It probably wasn't intentional given that the Tenda were originally called "Gumi" in Japanese.
The massive powerup Ness gets at the end of Magicant.
Or the one Poo gets in the form of a 'Message that has stat gains written on it.'
Toilet Humor: While there's a bit in the game itself, with its talking piles of vomit and the ability to root through trash cans, the game's infamous American ad campaign made it seem like this was all there was to the game.
Too Awesome to Use: Bags of Dragonite, which do massive damage to all enemies and of which there are only 6 in the game.
Too Dumb to Live: Exactly why does Buzz Buzz hover over to Lardna after Pokey and Picky are sent to their room?
Total Party Kill: High-level PK Flash spells as well as "glorious light" enemy attacks have a chance of inflicting instant death upon anyone who is hit. Both of these attacks hit the entire opposing party, and they have a fixed percentage chance of an instant kill regardless of the levels or stats of the entities involved. Naturally, mostbosses are immune to this effect, but your entire party can be wiped out instantly if your luck is bad enough when fighting enemies that can use these attacks.
Tortured Monster: Giygas. "...It hurts, ...it hurts... Ness, Ness, Ness, Ness, Ness... I'm... h... a... p... p... y..." It's even worse if you've played the game before this, because you'll understand why.
Trauma Inn: Averted; hotels restore HP and MP, but don't heal status effects — you have to go to a hospital for that.
Underground Monkey: Many later enemies are palette swaps of earlier ones, like the Manly Fish and the Manly Fish's Brother. Most notable with the Starmen.
Undisclosed Funds: Played straight in MOTHER 2, but not in EarthBound. During localization, for some reason a couple of vague references meaning roughly "a bajillion dollars" were changed to real numbers (Ness' family's debt to Pokey's family is "a hundred thousand dollars or more" and the Diamond "could pay off a million dollar debt easily").
Unobtanium: The Phase Converter runs on this — it's called Zexonyte and you get it backtracking to the meteorite.
The Maze: Talah Rama's monkey tunnels and the mole maze in the Dusty Dunes Desert, Brick Road's dunegon (and his insides as Dungeon Man), Moonside's teleportation NPCs, and the inside of the Stonehenge Base, just to name a few.
Wake-Up Call Boss: Frank Fly and his robot, Frankystein Mk II are the first real boss battle you have. They are not particularly difficult, but you do need to know what you are doing by that point.
Weaksauce Weakness: Buzz Buzz is a competent PSI user, and he can shield everyone in the party completely during the first boss. However, physically being not a bee, he dies when he gets swatted by Pokey's mom.
Mocked with the food stand in Happy Happy Village. 'Forget' to pay, and the salesman will hover over you and claim that you'll never know righteousness again.
The sign asking people not to trample the flowers — that can only be read when Ness is standing right in the flowerbed.
You Cannot Grasp the True Form: Trope Namer is Giygas' attacks, and Giygas itself probably qualifies as well. Mr. Saturn's speech is implied to be this, by virtue of the extremely weird font in which it's written.
Younger than They Look: Frank Fly is said to be about 20 years old in some of the literature based off the games. (Frank pegs himself as being younger than the legal drinking age if you talk to him during the endgame.)
Zillion-Dollar Bill: You are at one point given ten thousand dollars in cash and later an extremely valuable diamond, both of which you must use shortly afterwards to get the Runaway Five out of a bum contract. You'd think they'd have learned their lesson the first time.