This was ripped by Yuzi's GP21 Forums, but I wasn't sure if this was already posted somewhere.
If this isn't enough, I'll provide more meaning.
I played my Pokemon Crystal and used the Duplication Glitch, but before that I moved my Pokemon (Moving it without mail selection), so I can get my Poliwhirl's there to dupe. After getting about 37 Rare Candies, I put them back and leveled my Larvitar.
But when I did the RC to Pokemon dupe...
(That meant that I had to save, then level the Pokemon up and deposit it. Dupe that and you'll still have your RC's and your leveled Pokemon.)
...Larvitar was a Pupitar when I did this; I found out that instead of my nickname of Larvi, it changed to Whirl. I checked her stats and saw Poliwhirl's attacks instead of my Pupitar. It was not temporary although.
Now that I have my Tyranitar, now she will have these attacks:
Whirl (Tyranitar)
Level 100
Attacks: Surf, Ice Punch, Crunch, Earthquake
This also happened to my Mew, but her attacks wasn't changed.
Mhirl (Mew)
Level 100
Attacks: Thunderbolt, Psychic, Explosion ( Red ), Metronome
~~~~~
I also found out about duping with incorrect timing.
Steps on how this worked:
1 - Duplication. But try doing that with two Pokemon holding items that could evolve traded Pokemon.
How it happened to me:
Scyth and Jarrygo (That was my Scyther and Porygon) was deposited for duplication. But instead of the same old, I found that they were ?????'s filling 64% of the GBA window. Of course this wasn't temporary, but from looking at it (Pressing Withdraw) locked-up or restarted the game.
I took a couple of my own steps of trying to see this without locking/freezing the game.
1 - Changed the box.
2 - Selected move Pokemon without mail.
3 - ???
Some of the time, I have to press A up to 4 - 8 times to keep going down. But it didn't show my Jarrygo (Not looking at stats), instead there was a level 236 Kingdra.
To fix the game, try doing a battle or making a Pokemon learn a TM that you can get from shops. All the ?????'s in there will be "???????????????*insert attack / item here*". That way, I wouldn't have to keep restarting Crystal and get my Pokemon safely.
I traded one of the virus'd Pokemon to Blue, but the nickname and OT was all blank. Attacks was the same and I was safe doing this. But trading back affected the glitch once again.