Party item offset glitch
Performing the glitch
In certain circumstances, after selecting an item from the bag to use on a Pokémon, then selecting a Pokémon to use it on, it may be possible to quickly scroll up or down to an adjacent party slot. This article will use the term source Pokémon to refer to the one that was initially selected, and offset Pokémon for the one that was scrolled to afterward.
There are currently two known classes of items where this glitch has any effect.
By selecting a TM or TR and scrolling this way, the offset Pokémon will be checked for compatibility, without considering whether the source Pokémon is also compatible. If it is, the move will be taught to the offset Pokémon. Normally this would be no different than simply selecting the offset Pokémon in the first place for teaching the move, but there is one case where it makes a difference: if the offset Pokémon is an unhatched egg, it cannot normally be selected to learn TMs/TRs. However, by using it as the offset Pokémon in this glitch, it can learn the moves this way while still in the egg. If the egg already has four move slots filled, the usual prompt will come up to replace one of the existing moves, and the screen that appears while deciding on this will allow the player to see certain details of the egg, such as its stats, ability, and whether it's Shiny, without having to wait for it to hatch.
This glitch can also be used with the various sizes of Exp. Candy or the Rare Candy, as long as the player's bag contains only one copy of a particular size of candy; otherwise the attempt to scroll to an adjacent Pokémon will instead scroll through the quantities of how many candies the player wants to use at once.
After scrolling, the source Pokémon will always be the one that receives the experience points, so the scrolling will often not have any meaningful effect compared to simply using the item normally, on the source Pokémon in this case. However, if the experience gained this way causes the source Pokémon to level up, and it now meets the requirements for its evolution, and if both the source Pokémon and offset Pokémon are the exact same species and form, then the offset Pokémon will be the one that evolves instead.
This can be useful in that it doesn't require the offset Pokémon to also meet the normal threshold for evolution, and can thereby cause it to evolve while underleveled, or even while it's still in an unhatched egg, in which case the evolved species will be what eventually hatches. The event distributions for Eevee and Meowth with the Gigantamax flag, which are normally prevented from evolving, can evolve as the offset Pokémon this way, though they will lose the ability to Gigantamax thereafter. Male Combee or Salandit can also evolve using this glitch as long as the source Pokémon is a female version of the same species at a high enough level; doing so will cause the offset Pokémon to change gender and become female after it evolves. If both the source and offset Pokémon are Espurr of opposite genders, using this glitch with either of them will cause the offset Pokémon to switch gender as it evolves, as an alternative to forgoing the glitch and evolving it normally.
This glitch cannot be used to cause trade evolutions or stone evolutions to occur, because those evolutions aren't triggered by gaining levels. It also cannot cause, for instance, a non-Galarian Linoone to evolve by leveling up a Galarian Linoone, because those are not the same form.
If this glitch is used to cause a Nincada to evolve into Ninjask while it's still in an unhatched egg (and assuming there is a vacant party slot and at least one regular Poké Ball in the bag, as usual), another unhatched egg will be added to the party, this one containing a Shedinja.