The worst example I can think of for that would definitely be IG-88. I never once considered that as canon despite its standing.
Didn’t he (it?) download his mind into the Death Star II computer or something bizarre like that in the EU? Or was that another Skippy-type of story.
Also, even though Palatine is anything but a background character, I always hated his EU backstory. I always imagined him as some kind of ancient evil (which the movies kind of implied), but the EU just made him into some random sith from Naboo.