I Wanna Be the Guy/Shout Out

Warning: Contains spoilers:

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 * The game draws perhaps its heaviest overall influence from the Mega Man series. The intro screen is a direct callback to Mega Man 2, and the game features two bosses as well as an area with music and themes taken from that game. Additionally, two very common obstacles in the game - the vanishing blocks and the insta-kill spikes of doom - are ubiquitous mainstays of the MM series.
 * With the exception of the Guy, all of the bosses are taken from other games: Mother Brain, Mike Tyson, Dracula, Kraidgief, Mecha-Birdo, Bowser, Wart, Dr. Wily, and the Green Dragon. In almost all cases, these are infamously difficult bosses (particularly Tyson and Dracula).
 * The Guy's second form is a direct reference to the cyborg boss battle from Contra III, and the Mecha-Birdo fight also possibly references the aerial boss battle from the aforementioned game.
 * Several regular enemies qualify, as well, such as the birds from Ninja Gaiden.
 * Some of the screens are heavily based on areas of other games, such as Metroid, Mega Man, Ghosts N Goblins, The Legend of Zelda, and Castlevania''. Also, Tetris. And Breakout.
 * The Zelda area has a giant 8-bit Link who serves as a platform, but can kill you if you linger for too long.
 * You jumped into a sword, you retard!
 * The scrolling intro text (and the "8 units") are also Zelda-inspired.
 * Ryu makes an appearance and performs his trademark "Tatsumaki Sempukyaku" in order to open the path to the next area.
 * Much, if not all, of the BGMs used in-game are taken directly from other games. Kayin sure has good taste in game music. The save game selection screen? "Hunt a Soul". The music used in the first few game screens? "Home Sweet Grave". The Game Over music? "Might is Right but Tight". All from Guilty Gear Isuka, but a lot of people now associate them with IWBTG instead.
 * The music used for the road to The Guy's Castle is the theme from The Moon in Duck Tales, which was likely meant to lead the player to believe that a showdown with the eponymous object is imminent. This being I Wanna Be The Guy, however, the reality of the situation ends up being much worse.
 * The music used for The Guy's Castle is from Monty on the Run, an obscure Commodore 64 game with a kickass soundtrack and almost as much Nintendo Hard difficulty.
 * The music used for The Final Tower of The Guy? From E.V.O. Search For Eden''.
 * The Mecha-Birdo fight has the Ikaruga boss theme, complete with the WARNING screen in the intro.
 * Dracula's cutscene is straight out of Castlevania: Symphony of the Night, with the Kid saying Richter's lines.
 * In one of the few references not from a video game, there's a point in a tower climb where you are given two choices of paths to go up. Taking the wrong one results in death by FOOBY THE KAMIKAZE WATERMELON!! (Sadly, without his trademark fanfare.)