E3 Promo version Pikachu where the "Red Cheeks" Pikachu variant was distributed at the conference and the "Yellow Cheeks" was distributed via Nintendo Power.
epicThe "Red Cheeks" variant of Pikachu was released in the first (1st) and Shadowless versions.
The E3 Pikachu promotional cards were given away at the E3 (Electronic Entertainment Expo) events in Los Angeles in May 1999. It is unclear if the Red Cheeks E3 variant was included as a limited run intentionally, or if the change-over from red to yellow in the normal shadowless set affected the E3 promo prints unintentionally.
The "Yellow Cheeks" E3 stamp was distributed via Nintendo Power as is much more common.