here's a long, long page of weird, specific, and niche oddities in the pokemon games. the goal here is to list ones that aren't well known or generally dont matter except to super nerds (me, and maybe you).

technical quirks/trivia

sentret can learn tackle in gen 2, but cannot learn it in any generation after this! [src]

in gen 8, the move hold hands, an event only move introduced in x and y that does nothing, was dummied out in BDSP and ultimately removed.

in pokemon let's go: pikachu and eevee, a number of pokemon who don't learn amnesia in any other games can relearn it as a level 1 move (venusaur, clefairy, persian)! [src]

the starter pokemon in sun and moon have the highest collective base stats of all starter trios, at 320 each.

ditto is the only pokemon with a catch rate of 35, which changes in gen 3 and 4 to whatever ditto transforms into, and in gen 9, changes to 0 if it transforms a pokemon that's not its primary form (ie sky shaymin).[src]

deoxys is the only gen 3 pokemon that doesn't show up in pokeball pinball: ruby and sapphire [src]

bugs

in hgss, if the player uses the GB Sounds key item in cinnabar island or on route 20, there will be no music! [src]

in diamond and pearl, suction cups and sticky hold did not double the rate of fishing bites as intended due to a typo. this was corrected in platinum onwards.[src]

general trivia

buena's japanese name (aoi) makes the name of the points card (blue card) make sense, given that aoi means 'blue'!

the pokewalker is not the first pokemon addon to have an infrared function! that title goes to the pokemon mini.

in gen 3, the only ways to get dunsparce are either through pokemon colosseum or xd by trade, or by getting the national dex in firered and leafgreen and traveling to three isle port and going through a tunnel to a patch of isolated grass.

also in gen 3, the only way to get espeon or umbreon in firered and leafgreen is by trading to ruby, sapphire, or emerald, where there is a working time system.