Back before people had to spend 14.99 for a new memory card, saving your spot in a game required passwords. At some point or another, every kid tried putting their name into the password screen of their favorite video game, but since most kids aren't named "JXW67," it never really did anything. However, if your name happened to be Justin Bailey, and you were playing Metroid, you were in luck, as using that name as a password, coincidentally, gave you almost all the power ups and let you play as Samus in a bikini. I have to imagine whoever discovered this was much more excited than whoever futzed around with Mario for hours on end, only to discover that the minus world was just an endless water level.
Developers have a history of leaving unfinished business in their games, be they levels (like in Sonic 2), or features (like the infamous "Stop N Swop" from Rare's N64 days), leaving gamers to wistfully speculate on what could have been. Rarely, if ever, do they anger old white politicians, but GTA's hidden "Hot Coffee" mode was the exception. Running a mod on the game would allow the player to discover a minigame that was meant to be excluded from the game where CJ could have fully clothed sex with his girlfriend. Rockstar scrambled to make a patch, but it was too late, as politicians demanded the game receive an Adults-Only rating, ensuring that no one would ever buy, see it, it or speak of it ever again.
While the creation of an entirely separate quest that you can play upon finishing the original Legend of Zelda is hardly an accident, the events that led up to it's creation were a mistake. According to Toshihiko Nakago, Nintendo's President of Systems Research and Development, during the creation of the Legend of Zelda, director Takashi Tezuka accidentally only used half of the games data, mistakenly thinking that it was all the data that was available. Upon realizing this, they decided to make an entirely new second quest. It certainly wasn't the first time a game offered a replay after you beat the game, but it was much rarer for a game to offer entirely new levels, making it one of the most famous second quests in video gaming history.
At this point the meme has been overused and run into the ground so bad that it causes more anger than humor, but it's worth acknowledging that one of the most popular video game references of all time. The source is "Zero Wing" a Sega Genesis shooting game that was never released in the US, that contained a prologue with notoriously bad Engrish. While the intro is probably much less ridiculous in the original Japanese version, it's hard to imagine that anyone would be playing Zero Wing if someone hadn't set us up the meme (please don't send me hate mail).
Hardcore fans of fighting games and Tony Hawk enthusiasts probably can't imagine their favorite games without combos, fighting game combos actually originated by accident. It wasn't by design but a design flaw that players in Street Fighter II could cancel their last move animation with a new move, enabling the player to string together multiple moves to form combos.
The scantily clad women, however, are entirely on purpose.