There's an infamous underflow glitch in Pokemon R/B/Y.
The minimum level for a Pokemon in the game is lv. 2. If you get a lv. 1 Pokemon (need to use another cool glitch for that), and that Pokemon gains EXP that's not enough to hit lv. 2, underflow occurs and it gets billions of EXP, and hits lv. 100 immediately.
The minimum level for a Pokemon in the game is lv. 2. If you get a lv. 1 Pokemon (need to use another cool glitch for that), and that Pokemon gains EXP that's not enough to hit lv. 2, underflow occurs and it gets billions of EXP, and hits lv. 100 immediately.