Well I found this a bit rediculous too. Anybody ever watch "Pokemon 4ever"?
There is a bit of a Paradox, because in the movie Samuel Oak (Prof.Oak) is in Ilex Forest (or some other Forest.) and gets thrown into the future as a teenager when he is with Celebi. He goes to the present day when Ash and his friends are visiting the same forest. Sam has an adventure with them and draws Pikachu next to a Bellosom. So then he gets thrown back to his own time at the end of the Movie, and later grows up to be Prof.Oak. At the end of the movie you find out it was him because he opens up his drawing book and then sees the picture he drew those 50 some-odd years ago and he says: "Ah, it seems it was only yesterday..."
The reason this doesn't make any sense is because (as previously noted) Prof. Oak says in the first generation that there are 150 Pokemon (Minus Mew of course) so he was lying in some form. He would've had to known about the other 342 Pokemon, or at least 100 of them. It is almost impossible- well hell, it is impossible. But then again Ash never ages, and the Simpsons has more Paradoxes between episodes than there are cows in the American midwest. It would be nice though if it were explained... This is a good point and I'm glad somebody brought this up. I thought the better argument would be "Wait! There's a guy named Mr. Pokemon!?"