The front and back pages of Satoshi Tajiri's book "New Game Design", features what appear to be pre-release sprites from Pokemon Red and Green.

The international versions of Red & Blue are based on the Japanese version of Blue. Many changes were made in this version, including updated graphics and sound, as well as several bugfixes found in the Japanese versions of Red & Green.

Kingler's front-facing sprite erroneously depicts its claws as equal in size, when in reality the right claw is actually much smaller than the left.
Strangely enough, in the original Japanese Red and Green versions, the claws are their correct sizes. The claws are once again their correct sizes in Yellow version.
Additionally, the side that the larger claw appears on was changed after Generation I. The bigger claw became the left claw while the smaller claw became the right.

The original Red and Green Versions were supposed to have a female trainer along with the male trainers, Red and Blue. She was most likely removed due to hardware restrictions, but is still seen on the official strategy guide and artwork. The character was used as the character 'Green' in the comic and 'Leaf' in the remake, FireRed and LeafGreen.

Dratini's sprite as found in Red, Blue, and Green (both the original Japanese release and the updated International release with different sprites) Versions has stripes on its belly. These stripes were removed in Yellow Version and have not reappeared since.

There is a man who is located at Cinnabar Island's Pokemon Laboratory who requests a Raichu in order to trade for an Electrode. After the trade is complete, the man then says 'The RAICHU you traded to me went and evolved!'

This is a translation error where the Japanese version of Pokemon Blue has a Machoke in place of Raichu where as Pokemon Red and Green have Raichu.

The Pokémon sprites were changed in Japan for their special edition of the game, Pokémon Blue. Originally, Pokémon Red and Green had different sprites to those of the English release. These new sprites were used in the international release of the game. It's widely agreed that the revised version in Pokémon Red & Blue are far superior to those in Pokémon Green, where a lot of proportions were out of place.