The Reflect/Light Screen stat overflow glitch is a natural glitch in Pokémon Red, Blue, and Yellow.
When a Pokémon with a Defense or Special stat of 512 or 513 uses Reflect or Light Screen respectively, having it get hit with a physical move or special move (respectively) will freeze the game. If its Defense or Special stat was a higher value, its stat may be treated as if it was much lower during damage calculation.
This glitch happens because stats can never be a value greater than 1023. If the Defense or Special stat is higher than this the value will overflow past 1023 and wrap around to a lower value. For stats 1024 and 1026 the game will attempt to divide by zero when calculating the damage, resulting in the game locking up.