My copy of Pokemon Diamond was on this game cartridge thing called a TTDS. It looks like a regular DS game cartridge, except it's white instead of dark gray, and it has a micro-USB port on the top. It had that and 60-something other games on it, and many of them had some amount of cheats (kinda like an action replay, I think). Diamond had a lot of cheats I could turn on and off when I pleased- walk through walls, make all wild encounters shiny Pokemon, max out any item, like Rare Candies and TMs, make any Pokemon appear in the wild at either level 1, 10, 50, or 99. (Random fact- did you know that every time you encounter a wild Dialga/Palkia or Arceus, it'll do the little cutscene and play its battle theme?) There might have been other things too, but these were the main ones I used. My brother also had a TTDS and he showed me how to turn them on and change them. I was at around the 4th gym and pretty much always left the cheats on. A completely unintentional and unexplainable glitch happened to my starter Empoleon- it started off as a regular Empoleon, but somehow it became shiny. I still have no idea how, 8+ years later.
As expected of young me messing around with the glitches, I was fascinated by glitches in video games. I remember I'd watch videos of people showing glitches in Pokemon games. I was bummed when I found out there weren't nearly as many glitches in any of the other games I played. Other than Diamond, I haven't done much glitching in any games. I have done those PKHex things- in White 2 (before the GTS shut down) and Sun. For White 2, I did it for Pokemon I was too lazy to catch myself and trade evos. I made some really stupid things- I gave a Lopunny the ability Multitype, but I think it only works for Arceus so she basically has a useless ability. A Conkeldurr with the ability Compound Eyes and all one hit KO moves, named Conkelderp. A Wonder Guard Drapion (which I wish I thought of Wonder Guard Spiritomb or Sableye at the time...) All the Hitmons with the original trainer's name as "Douche" (I was an immature kid) For Sun, I only used it a few times, for battle-ready mons I can't breed myself. (Trying to breed for perfect IVs and the correct nature is too difficult for me, and I'm lazy)
There was also a GTS exploitation glitch back in gen 5 where you could get a random Pokemon out of the GTS. IDK how to explain it since it's been a while, but I think it took out a random Pokemon that was being offered to trade. I only remember getting a Miltank, a Registeel, and 2 Regigigas. Sometimes I got the same Pokemon twice. I also remember a Youtuber who offered out hacked perfect EVs and IVs legendary Pokemon. (That's the only reason I have any of the legendaries or mythicals in my Pokedex tbh.) I can tell some of them are hacked because when I was transferring all of my Pokemon up to Y, some Pokemon that had illegal moves and such weren't allowed to go up. (Things like my Wonder Guard Drapion are forever trapped in White 2.) They all had legal moves and abilities, but I noticed the catch location for the Pokemon was the "illegal" part: Kanto legendaries said they were from "Kanto" instead of "traveled through time and space from the Kanto region", the same goes for the other regions.
Also I don't think this counts as a glitch, but the internet can still be accessed through a certain method, even though Wifi services for DS systems were discontinued in 2014. So the GTS can still be accessed today in 2022, and you can get old Mystery Gifts from 2013.