It's only inevitable for a game like Super Smash Bros. to have its glitches emerge on a regular basis due to the amount of playtime that it is subjected to by millions of players everyday. Each match consists of a multitude of factors, triggering all kinds of events. In fact, a strange outcome of becoming 'Fire Kirby' has recently been discovered by YouTuber Dalas Onatop, which YouTuber Master0fHyrule has made a little more public with their video explaining the glitch.
The glitch involves two players, with one of these players using Kirby and the other being either Kirby or King Dedede. The glitch can be executed by one player performing Kirby/Dedede's Inhale special. While this player is inhaling, the other player (using Kirby) must perform their side special, the Hammer Flip. However, this player must not swing the hammer until just before being inhaled by the other player's Kirby/Dedede - leading to the full animation being cut-off.
If performed correctly (the timing is quite precise), the inhaled Kirby should emerge into a ball of fire (or Fire Kirby as many seem to call it) once escaped from the inhaling character. It's important to note that the fire surrounding Kirby is purely aesthetic, and does not do any damage nor have any side effects. However, it's super cool fighting in this state, and can easily be removed (if you somehow grow tired of it) by performing the Hammer Flip special again, or being KO'd.
Check out the video below to see it in action, and let us know if you've noticed anything should you try this glitch out for yourself.