A bad ending[conjectural title] (or false ending[conjectural title]) is any conclusion to a story in a given game that is sub-optimal for the hero(es), and not part of the canon. These endings (if they exist) are typically shown to the player if the game is either lost, or completed without the necessary prerequisite objectives to obtain the true ending. Technically speaking, every 'Game Over' screen is a form of bad ending - because it implies that Kirby or whoever else is serving the role of main protagonist failed somewhere in their adventure - but this article will mainly focus on the other type of bad ending, where the game is seemingly completed.
Standard Vs. Full Completion
Before specific examples of bad endings are cited, it is important to distinguish between a proper bad ending with credits roll and/or a cutscene, and an otherwise true ending where not all in-game objectives are met. The latter is characterized by getting the same ending regardless of how many of the important collectables were picked up along the way. An example of this would be the Energy Spheres from Kirby's Return to Dream Land. While the canon-telling of the story has Kirby and his friends collecting all 120 of them, this is not necessary to reach the proper ending of the story mode. Collecting all 120 in turn will not change the ending cutscenes.
Examples by title
Kirby's Dream Land 2
Once Dedede goes down, Kirby poses in triumph. From here, a cutscene plays where Kirby walks back home in the rain with a Parasol in hand, as his three Animal Friends follow him closely behind. Below this scene, an enemy line-up is shown to the player, containing every enemy and Boss in the game. At the end of this line-up, a silhouette of Dark Matter is shown, with the name !?, implying that something was missed. From here, The End is displayed, as the party walks off-screen. Kine the sunfish trails behind them, and pauses in the middle of the screen for a brief moment. He looks up, and a ? is added to the end text. From here, Kine waddles off-stage, and the game resets.
Kirby's Block Ball
If any of the first ten levels are completed without having met the border line score, then a bad ending will play out after defeating Brobo in level 10. Kirby will hop down from a cloud and reach the ground, but find nothing there. After looking around (presumably for the fountain), he will hoist a traveler's sack over his shoulder and walk off to the right, as 'THE END' appears overhead. Just after Kirby walks off-screen, however, a shadow of King Dedede's image appears in the background sky. Kirby returns and makes one more glance back and a question mark appears over his head, which is then transferred onto the text to spell 'THE END?'. From there, another text crawl appears which reads 'LET'S TRY AGAIN!'.
Kirby's Dream Land 3
Once King Dedede goes down, Kirby hops off his Animal Friend (if he was riding one), jumps a few times in the air, and poses in triumph, performing the same animations from Kirby's Dream Land 2. Afterwards, the game transitions to a new scene where Kirby and Gooey are walking back home in a grassy field in the daytime. Additionally, like in Kirby's Dream Land 2, a list of all the enemies and Mid-Bosses are shown to the player along the way. At the end, the camera pans upward to show a large black circle looming in the sky, with the caption of ?. This is the yet-undefeated Zero. From here, the game resets.
Kirby 64: The Crystal Shards
Kirby and his team watch as the crystal banishes the plague of Dark Matter from Ripple Star's skies, where it then flies off into space, disappearing with a twinkle. Kirby and his friends celebrate this as fireworks fill the sky. Ribbon and the Queen of Ripple Star see the party off, giving them a transport ship to take them home. Before Kirby leaves, he shakes hands with Ribbon. Once the ship carries them away, the Queen turns to face the camera and smirks, her changing eye color implying that she remains possessed by Dark Matter.
The credits sequence afterward shows concept art sketches from the game's development. Unlike the ones in the true ending, these sketches are not colored in, and 0²'s artwork is a sketch of its form before the battle starts. From there, the game resets.
Kirby Star Allies
Exclusive to the Heroes in Another Dimension mode, a bad ending plays out if fewer than 100 Friend Hearts were collected prior to defeating The Three Mage-Sisters. In this ending, Kirby and his allies simply walk off after they defeat the Mage-Sisters, leaving them and Hyness in a pile on the floor. While the Mage-Sisters lay there unconscious, a dark, ominous aura begins to emanate from them, while the bad ending jingle from Kirby 64: The Crystal Shards plays to a quick fade-out.
Having this ending play out will still unlock the Soul Melter EX difficulty for The Ultimate Choice (provided Soul Melter was already completed beforehand), but will not unlock the Three Mage-Sisters as a Dream Friend, nor will it unlock the final three Celebration Pictures. Additionally, King Dedede appears in the credits game instead of Marx, the ending caption reads "The End...?", and in the Theater, the credits movie is titled "The Happiest Curtain Call?".