Victory Fakeout

""You defeated your arch rival the sinister Warlock. Peaceful days will now return to Vampires and Monsters.""

- Epilogue Text, Monster in My Pocket

"Warlock: Ha! Ha! Ha! ... I have returned! Vampire: ...Warlock!... Warlock: You thought you defeated me. I'm much stronger than you expected. Now I will destroy you!! Vampire: ...What!!..."

- Monster in My Pocket, several seconds later

So, you've defeated the Final Boss after a long and hard battle, the good old victory music that played after defeating the other bosses kicks in, and your character does his usual dance...

But then, the music suddenly grinds to a halt as the boss suddenly gets back up again and reveals his One-Winged Angel form. Your character stops his dance, realising he now has to fight a much bigger, badder form of the boss he was fighting. This is when a boss pulls a Victory Fakeout.

Subtrope of Hope Spot and Sequential Boss. Compare Your Princess Is in Another Castle and Trick Boss. If the player character dies during an actual victory scene, it's a Kaizo Trap.

Action Adventure

 * In The Legend of Zelda: The Wind Waker, Puppet Ganon pulls this on the player after defeating his first form.
 * Happens twice in The Legend of Zelda: Twilight Princess. First when fighting Stal-lord, and then again against Arghoma.
 * In Okami, after defeating Yami's fourth form, Amaterasu begins her victory howl, only to get taken out midway through by Yami's fifth and final form.

Beat 'em Up

 * Carnage returns to attack the player's Spider-Man and Venom during the credits sequence in the Maximum Carnage game.

Fighting Games

 * Gill from Street Fighter III may count, if you're unprepared for his infamous resurrection ability.
 * In Marvel vs. Capcom 2, Abyss does this twice.
 * Then in Tatsunoko vs. Capcom, Yami does the same thing, except on those occasions your health is restored.

First Person Shooters

 * In the Borderlands DLC, The Zombie Island of Doctor Ned, you go toe-to-toe with the titular Doctor... then you kill him in less than half a minute, get an achievement, and the credits roll. Then the real boss, "Undead Ned" rips his way through the screen, laughs at the player, and the fight resumes.

Platform Games

 * The page quotes comes from Monster in My Pocket, an old NES game by Konami, when you defeat Warlock. The usual stage cleared screen is shown, and then the ending begins to play, but then Warlock suddenly comes back and you then fight him on a giant televison screen. After beating him again, your character simpily exclaims "Yeah..." and then the credits roll.
 * Subverted in Donkey Kong Country, where the fake kredits during the final battle is blatently obvious, such as enemy names in place of developer names, and the fact the final battle theme continues to play regardless.
 * In Kirby's Epic Yarn, after beating Yin-Yarn, Kirby and Prince Fluff do their usual victory dance, but then the large needles he left behind come to life to knit a larger, mechanical version of Yin-Yarn. Cue epic Humongous Mecha battle.
 * Likewise, a similar thing happens in Kirby's Return to Dream Land, after the Disc One Final Boss Landia is defeated. Magolor seizes the dragon's Master Crown and reveals that he had been plotting a Batman Gambit to rule the universe all along, with Kirby and co. as the Unwitting Pawns.
 * For an example with the victory music playing and dying down when the boss arises, there's the battle against yet another Disc One Final Boss, the Grand Doomer. The boss stage is just a room with the final piece of Magolor's broken ship, until you attempt to get it, and the piece transforms into the boss. Why, did you think you were going to get that mast without even having to fight?
 * After defeating Bowser in New Super Mario Bros. Wii, Mario (and the other players if playing multiplayer) looks up at Princess Peach, only to be revealed to be Kamek in disguise, who then casts a spell on Bowser, making him colossal.
 * Baby Bowser pulls the same stunt even earlier in Super Mario World 2, thanks again to Kamek and his Make My Monster Grow trick.
 * At the end of Super Mario Galaxy 2, Bowser first appears to have been flung into space as a result of Mario ground-pounding his own meteorites at him, but just right before he can even get the last Grand Star, Bowser flies back up, eats the Grand Star, and becomes huge again, and as a result Mario had to ground-pound more meteorites at him to finish him off.
 * Super Mario 3D Land did this twice: After the first battle against Bowser, Mario runs up to Peach's chamber to find a cardboard cutout of the Princess, and then sees Bowser carrying the captive Peach, who then mocks him and jumps off the tower. This also happens again during the real final battle, where Mario knocks Bowser into a lava pit (after he throws barrels at Mario)... ...and then Bowser jumps back up and starts chasing him.
 * Happens in the Final Boss battle of I Wanna Be the Guy.
 * In Sparkster on the Sega Mega Drive/Genesis, after beating the Final Boss, he explodes and sinks below the ground as the screen fades to black, but then he comes back again in a much bigger size.
 * Used by Mother Brain in Super Metroid after "her" cutscene death.
 * Dr. Wily in Mega Man 3, Sigma in the first Mega Man X game, and Elipzo in Mega Man Zero 2 are particularly clear-cut examples.

Puzzle Games

 * In Zuma's Revenge, one shot on the false Zhaka Mu will drop him. The real one interrupts the fake end credits, and he's much harder to beat.

Role-Playing Games

 * In Super Mario RPG, after enduring a two-phase Boss Battle against the volcano boss, the Axem Rangers show up and swipe the sixth Star Piece out of your hands right in the middle of claiming it.
 * In Paper Mario, after knocking out Lava Piranha, the victory theme is about to kick in when the ground shakes and the Lava Pirahna gets back up for another round, and this time it is on fire.
 * The fact that the Star Points don't get added up into your total hints to this.
 * Cortez the Pirate Ghost returns from his defeat sequence twice in Paper Mario: The Thousand-Year Door, in a stage that's something of a successor to Lava Piranha's world in the first game.
 * In Episode II, the first fight against the Patriarch actually goes to the post-battle XP screen, then breaks to send you into the battle against the next form.
 * Simularly, in Final Fantasy VII, after defeating Bizzaro Sephiroth, the screen fades to black and it appears the post-battle XP screen is going to appear, but instead the game is loading the One-Winged Angel battle.
 * Ultimecia does this after beating her Fusion Dance form. The HUD disapears and the music stops, and even throws in a fake Technicolor Death for good measure.
 * In Seiken Densetsu 3, one of the three potential final bosses, Archdemon, appears to explode and die in a large explosion after defeating him, like the other bosses, only for him to reappear in his next form.