What's the deal with our beloved English language where a word and what would appear to be its opposite actually mean the same thing? For example, there is ravel. Then there is unravel. You would think by adding un as a prefix to the word that it would negate the root word's meaning. When the scarf you're knitting begins to ravel, it means that it is coming undone. When your knitting begins to unravel, it SHOULD mean that it is somehow magically coming back together, but oh no, it's not so. There's just more of it coming undone. It's like the Law of Entropy on steriods. In this case, you really are damned if you do and damned if you don't. One way or the other, you're going to end up with a pile of yarn and no scarf. Ravel and unravel mean exactly the same thing.
How about sever and dissever? Should you ever accidentally sever your finger, not only do you want to get to the hospital quickly with your injured hand and the severed finger, you also want to make sure that instead of instructing the hospital staff to dissever your finger, you make sure they know that you want them to sew the already severed finger back on. Otherwise you might end up with more than one missing digit. While that may be a little far fetched, it does make you scratch your head and say, "What the ...?"
The same is true with flammable and inflammable. In this case, the confusion is not merely annoying or uncomfortable, it could be downright dangerous and potentially deadly. If a material is flammable, you definitely want to take precautions and keep it away from a source of heat. Just make sure you don't fall into the word trick of thinking that a material labelled inflammable means that it won't catch fire. Otherwise your world might go up in smoke because the words mean exactly the same thing. Fortunately the word inflammable has become mostly obsolete, which I suspect had to do with the internal infernal conflict of the words in question. Still it makes you stop and wonder, "Who thunk of that?"