Experience/Core series/Experience underflow glitch

In Generation I and Generation II, level 1 Pokémon in the Medium Slow group were calculated to have -54 experience points. However, due to the use of, the game interpreted this value as 16,777,162 experience points. If a level 1 Pokémon with negative experience points completed a battle without gaining enough experience points to reach 0 or higher, the game, attempting to determine its level based on the number of experience points it had, would consider it to be at level 100 (having gone well over the amount required to reach this level), causing it to instantaneously jump to this level.

It is due in-part to this bug that no level 1 Pokémon could be found in the wild without abusing a glitch or hacking the game in Generation I and Generation II. It is also partially for this reason that Pokémon hatched from Eggs at level 5 when Eggs were introduced in Generation II (although level 2-4 Pokémon could be found in the wild).

The glitch was fixed in Generation III, which uses a lookup table, rather than a formula, to determine experience requirements (this is also why the two new functions introduced in that generation are able to be piecewise functions). Despite this, s still hatched at level 5, and the lowest level that Pokémon could be found at in the wild was still level 2. Generation IV finally featured the first instance of legitimate level 1 Pokémon, where s hatch at level 1 and level 1 Pokémon can be found in the wild.

Additionally for Generation I, any Pokémon in the Medium Slow group that was deposited in the PC at Level 1 will cause the game to freeze upon attempting to withdraw it, causing it to be permanently stuck in the PC due to a glitch. 