We're lucky that Super Smash Bros. Ultimate was released on a system where the games can be patched, as yet another game-breaking bug has been discovered, but it only occurs under very specific circumstances.
The glitch was discovered by two Reddit users named FlamingOranges and VocalEcho, who revealed it in a thread that detailed how you can cause Super Smash Bros. Ultimate to shut down in a battle between Duck Hunt Duo and the Mii Swordfighter.
In order to cause the glitch, the Mii Swordfighter requires a very specific set of moves: Shuriken, Gale Stab, Stone Scabbard, and Reversal Slash.
If a Mii Swordfighter enters a battle with a Duck Hunt Duo using that configuration of moves and if they both use Wild Gunman and Gale Stab at the same time, then Super Smash Bros. Ultimate will crash and the system will return to the Nintendo Switch's main menu.
The Duck Hunt Duo/Mii Swordfighter glitch isn't the first to be discovered in Super Smash Bros. Ultimate, as the fans have already found a nasty bug that causes Isabelle to keep summoning Assist Trophy characters after the item has only been used once.
It's not surprising that Super Smash Bros. Ultimate has launched with glitches, as the game had a tight 2018 holiday season deadline to hit. The culture of patching games is such that the developers likely thought that any minor glitches would be easily forgiven, so long as the game works as a whole, and that any problems could be fixed over time with regular updates.
A new Super Smash Bros. Ultimate patch is due to be released sometime before February 1st, but the discovery of this glitch was so recent that the developers may not have time to fix it in the next update.
The fact that Joker from Persona 5 is due to appear in the game soon means that this very specific glitch would likely be dealt with before he arrives in Super Smash Bros. Ultimate.