Somewhat related to this question, I am curious to know what words in English would seem to be opposites at first blush but are in fact synonyms?
Immediately I can think of flammable and inflammable.
some related examples in slang. These use the same word, but mean the opposite:
Valuable and invaluable is the only example that comes to mind
Though they are of different origin, I would go for 'genius' and 'ingenious', perhaps?
I also have some points to raise from previous answers:
Irregardless is most likely from a combination of 'irrespective' and 'regardless', which are synonyms of each other. Therefore 'irregardless' is seen as an erroneous construction by many, as Sam pointed out.
Also, 'ravel' and 'unravel' are not the same. The phrasal verb 'ravel out' is synonymous with 'unravel' but alone, 'ravel' means to complicate, while 'unravel' in that sense would mean 'make clear something that was complicated'.
The answer to this one might be "false friends." The term refers to cases where a word in a foreign language is deceptively similar to one in your native tongue and leads you thus astray. Here the case is a bit different but I could see extending the sense a bit.
Another example in the same class is the word "enervate," where the Latinate prefix (as with "in" in "inflammable") has a meaning that leads the unknowing speaker down the wrong path.