I've been to a couple, I can remember my first, which was in Toys R Us in Cardiff back in 2006, where you could download Mew onto your FRLG and try out Pokemon Battle Revolution. They had a competition for people who came dressed as pokemon and there was such a large queue I remember being stood next to a girl dressed as a Lucario and someone I think was dressed as Cacturne. They gave out some goodie bags and I remember trying to buy a load of pokemon toys whilst trying not to be embarrassed about it because I went with a friend who didn't like pokemon but knew we were going for a pokemon event.
I remember an Arceus one in GAME as well just before HGSS and you could play the demo for those games which was great, and also the Shaymin event which again was at GAME but also seemed much busier.
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