To the best of my knowledge, English is the only language where "inflammable" meant "can catch fire" but fifty years later, meant "cannot catch fire" because of colloquial use. Mark my words. Fifty years from now, "literally" will mean "figuratively."
There are TONS of words like that, I'm frustrated I can't think of any off the top of my head other than "terrific." What's amazing is that there are English linguists who track when words first came into use, how their popularity and use fluctuates over time, etc. I'm sure they're doing a very necessary job, but at first glance, it seems trivial.
My favorite example is "nimrod." Nimrod in the Old Testament was a great hunter archetype. Fast forward to the 1960s, and Bugs Bunny uses it to insult Elmer Fudd--basically calling him a great hunter sarcastically. Too few people understood the reference, but gleaned the idea Bugs was going for, i.e. to make fun of Elmer. From this, nimrod came to mean "dummy."