Infinite Assist Trophy glitch
The infinite Assist Trophy glitch was a glitch in early versions of Super Smash Bros. Ultimate that caused Isabelle to repeatedly summon Assist Trophies, with no control over this action until she got hit.
In order to perform the glitch, two players must be playing as Isabelle and use Fishing Rod at the same time to grab on to an Assist Trophy. If the glitch is performed correctly (one Isabelle will reel in the Assist Trophy while the other's line is still hooked onto it), the player that grabbed a hold of the Assist Trophy will continuously summon that Assist Trophy until they get hit, pushed into a blast zone, or KO'd in some other way. If one player is significantly farther or closer to the Assist Trophy than the other player, the glitch will not work. Additionally, certain Assist Trophies do not spawn more than once, such as the Squid Sisters.
The glitch also works with a handful of other items: for example, Warp Stars will cause the player who picked it up to be stuck grabbing on to it, and no attacks will damage them, while Hammers or Golden Hammers will function as normal but will deal no damage, and the player will not be KO'd by the bottom blast line. The only way to get rid of this glitch is by ending the match, or resetting positions if the glitch is done in Training Mode.
The glitch was patched with update 2.0.0.