The glitch is a fairly situational one and needs a few actions to fall into place for it to work, but it revolves around one fighter: Ice Climbers. Squall Hammer right when they're grabbed by an enemy, they'll free themselves from the grab animation and every grab attempt after it by rendering grabs unusable for the remainder of the match
Competitive Super Smash Bros. player and Twitter user Shared the video below that shows what happens when the Ice Climbers escape from a grab using that ability. Luckily, you can grab the Ice Climbers with his head 5000, but as soon as they break free, they can't grab them anymore.
Apparently if IC's cancel your grab with a page b, they can't be grabbed for the rest of the match … pic.twitter.com/AjEn23dMGe
̵1; Butter (@KY_Butter) December 22, 2018
Ice Climbers have a history of causing problems in Super Smash Bros. prior to Ultimate's release, the character action a bit differently than others since there are two people wrapped in one fighter, but this could be a problematic one for anyone up against an Ice Climbers user. Many fighters rely on grabs to set up their combos, so that part of their kit could severely hinder certain characters. Seeing this happen in an actual game sounds like it's a rare occurrence though given it's difficult to replicate, according to the player who found the glitch. Butter described as having "really strict timing" but said it affects both tether grabs like Luigi's as well as normal, up-close grabs. It also affects the Ice Climbers and removes their ability to grab their opponent, too, so they will not benefit from the glitch quite as much as they are part of their arsenal.
ALSO when this glitch is activated NO ONE can grab , not the Ice Climbers not even playing the glitch.
– Butter (@KY_Butter) December 22, 2018
Bugs and glitches have been found occasionally in Super Smash Bros. Ultimate since it released, so this is far from the first that has been discovered. Players found another bug not long ago that resulted in the game crashing if Isabelle and Villager went up against each other and played a game of catch for a while.