I've always been struck when I see some unusual words that have the negative prefix removed. Some of the base words are just never used. There's the always fun flammable/inflammable which mean the same thing. But there's just so many more fun ones! My favourite examples of removing the negative prefix:
dolent, ept, ert, ertia, fect, fidel, maculate, occuous, sipid, solent. I admire those that are ept with finances and have a solent and sipid attitude.
A few others:
advertant, chalant, conscionable, continent, cumbent, defatigable, demnify, dentured, seemly, souciant, toward.
I wonder a bit about the history of these words. Have they always been the "negative" first, or did they evolve from the non negative case? Most of them seem to come from negating a latin base. Maybe they never had the base word only in english.
And how did the rules about whether il/im/in/un should be use evolve? They kind of make sense for the prefix of consonent bases to use the same consonent of the base word for the prefix end, but what about the vowels? The 'n' ending seems to be easier to say, but then why is illadvised rather than inadvised used? Ah, but you say "ill" means bad in illadvised so it's not the same as not advised. Does that mean that somebody could be un/inadvised? And that means the "negative" prefix is context sensitive whether it's a "not" or a "bad". Neat!