English is not a fixed language so with time, words tend to come in more often and become acceptable. These words are commonly called pseudoatonmys
FLAMMABLE/INFLAMMABLE Looking at the above words, first thing that comes to the mind is, one catches fire and the other doesn't. Actually they mean the same thing.
BAR/DEBAR Bar being the act of prohibition, funny enough debar also means the same thing.
REGARDLESS/IRREGARDLESS Regardless, prevailing against odds and circumstances. Irregardless is likely originated from a combination of 'irrespective' and 'regardless' and thus became accepted.
HABITABLE/INHABITABLE Habitable simple implies Suitable to live in. You would think inhabitable was any different, but it means the same thing.
GENIUS/INGENIOUS Everyone has probably heard of genius. Clever,inventive person, and exceptional intellect. Ingenious also implies the same thing.
TERMINATE/EXTERMINATE To terminate, the act of bringing to an end of something. Your guess would be with the prefix of 'ex' there would be difference but it does almost mean the same thing.
RESTIVE/RESTLESS Your first opinion of restive would be in a state of rest, but restive actually means being unableto stay silent or still, thereby restlessness.
VALUABLE/INVALUABLE Valuable means of great worth. Invaluable also means the same thing, being useful and indispensable.