As discussed in the Visual Discrepancies in the EU thread, here's a listing of some or the more notable visual discrepancies in the EU:

CHARACTERS
Airen Cracken - WEG RPG vs CCG/Visual Dictionary
Borsk Fey'lya (see Bothans below)
Corran Horn - I, Jedi cover vs goatee version vs Michael Stackpole CCG
Ghent - WEG RPG vs comic
Guri - red and green outfit vs blue and gold outfit
Judder Page - WEG RPG vs Rebellion vs CCG vs comic disguise
Keyan Farlander - Galaxy trading card vs Essential Guide
Maarek Steele - Galaxy trading card vs Gamer
Mara Jade (Arica) - comic version vs CCG version
Tionne - pupils or no pupils
Vanden Willard - Bob Hudsol mistaken identity

ALIENS AND CREATURES
Annoo-dat - blue (Droids cartoon) vs orange (Jango Fett comic) species
Bothans - WEG RPG version vs Specter of the Past cover
gundark - aquatic and "long-necked" subspecies
hawkbat - young vs mature
krayt dragon - greater krayt dragon subspecies
Kwi (Blue Desert People) - WEG RPG vs comic
nerf - Alderaanian mountain nerf subspecies
Noghri - WEG RPG version vs Heir to the Empire comic version vs WOTC RPG version
womp rat - mutant variation
ysalamiri - WEG RPG version vs salamander-like version

TECHNOLOGY
Basilisk war droid - droid mount Tales of the Jedi comic vs ship Kngihts of the Old Republic
dark trooper -sorting out the various stages and sizes
Rand Ecliptic - X-Wing comic vs Darklighter comic
spacetrooper - WEG RPG version vs Heir to the Empire comic version

LOCATIONS
Kessel - Early Adventures comics vs barren rock

CODE KEY
CCG = Deipher Collectible Card Game
RPG = Roleplaying Game
WEG = West End Games
WOTC = Wizards of the Coast