There are many words in the English language that we take for granted when we were just plain readers, but once we turn to serious writing we find out nuances we were unaware of. I will just show a few of the ones I learned about.
Flammable, Inflammable -- All my life it never occurred to me to even think about these words. I always assumed that flammable meant something would burn while inflammable meant the opposite. Nope, they both mean the same thing. I really don't like having confusing pairs like this!
Further/Farther -- Farther seems to be relatively new compared to further. Many Brits still prefer to only use further; for instance, check out Lord of the Rings and you will never find farther in it. Modern English now prefers to use farther for instances of physical distance.
Gray/Grey -- It is again strange to have two such similar words mean exactly the same thing. My feeling was that it was simply a matter of taste, so I began to write my novel using grey. Later I ran into a longer word that incorporated gray into it, so for consistency I had to change all of my greys to grays.
Amid/Amidst, Among/Amongst -- I never thought about these prior to taking up writing. As far as I can tell, these are purely a matter of taste, no difference in meaning. As with Gray/Grey and Further/Farther, I believe in being consistent in usage. It constantly amazes me to see even famous authors having farther in some spots and further in others when they all referred to physical distances.