Uh, first glitch was not something I intentionally activated, one time, I was playing Platinum and I was watching the TV when suddenly the text was garbage (as in, it wasn't words, but weird symbols, no, not the Unown symbols, btw) for a few seconds. But, I definitely experienced a different glitch in Diamond before this, but I don't remember well.
Also, I experienced the Pokemon transforming into something else glitch in Mystery Dungeon, and no, I don't mean a Ditto using transform, I mean two of the same Pokemon standing next to eachother and rapidly changing from one color to the next and saying it was a different Pokemon when I defeated it. But, the first time it happened, a Togepi became a yellow Ratata.
The only glitch I intentionally ever did was the cloning glitch in Emerald. I still remember losing my Groudon and Kyogre to this glitch.
But, a lot of the glitches I've had happen to me were just random. Like, I had the Acid Rain glitch happen in the cemetery in Sinnoh once. (In Platinum).
And... I believe I told the story about my scary, barely playable Pearl that's full of scary glitches and freezes, right? One of the first "scary" ones I encountered was I was battling a Hiker and suddenly... "Geodude was buffeted by the ---!" and the game froze...