There were a lot of errors in the original G/S/C sprites. Let me name them (I'm looking at a sprite sheet as I'm typing this):
Chikorita (Wrong color)
Meganium (Pink leaves drawn incorrectly)
Crocnaw (Too green)
Feraligatr (Too green)
Pidgey (They're not supposed to be pink)
Pidgeotto (They're not supposed to be pink)
Pidgeot (They're not supposed to be pink)
Pichu (Cheek spots are supposed to be pink, not red)
Pikachu (backsprite mistake: The tail is facing the wrong direction)
Spinarak (Wrong color. Though it could be argued that this was done intentionally to make it appear at night time)
Ampharos (Ear stripes incorrect)
Skiploom (Tips of ears shouldn't be yellow)
Poliwhirl (It should have mittens, not gloves. This mistake remained in R/S)
Bellossom (WTF PINK!?!?!?)
Pineco (Completely wrong color)
Forretress (Completely wrong color)
Sunkern (It should be yellow, not green)
Aipom (Colors are way too dark)
Machop (Should be greenish-blue, not brown)
Machoke (Should be purple, not grayish brown)
Machamp (Should be greyish-blue, not brown)
Smoochum (IT SIMPLY LOOKS UGLY)
Farfetched (Too fat, there shouldn't be stripes on wings, and the wings are drawn rather incorrectly)
Xatu (There should not be an "eye" on the wing)
Tentacool (Tentacles shouldn't be blue, though this was done because of color restrictions)
Tentacruel (Tentacles shouldn't be blue, though this was done because of color restrictions)
Starmie (Gem in center should be red, not purple. Though this was done because of color restrictions)
Shellder (Wrong color)
Remoraid (Wrong color)
Octillery (Eye should be white, not yellow)
Lanturn (Wtf is up with it's "hands"?)
Tangela (Wrong color)
Kingdra (Too dark)
Gligar (Wtf is up with the blue tongue?
...)
Skarmory (It should have red wings)
Kangaskhan (Wrong color)
Magcargo (Wrong color)
Sneasel (Wrong color, incorrect "ears" and "wings")
Aerodactyl (Wrong color)
Raikou (Obviously)
Entei (Obviously)
Tyranitar (The head is drawn slightly incorrectly)
Most of these were fixed in Crystal.