A while back, when we learnt how to remove the caffeine from coffee beans, we coined the word decaffeinated to denote coffee that's had the caffeine taken out.
I've noticed more and more recently, as decaffeinated coffee has increased in popularity and availability, that the term caffeinated has crept in to describe coffee that has not been decaffeinated.
Does this make sense? The -ate suffix surely denotes an active process that is applied to something. So one can decaffeinate by removing the caffeine; but unless one is attempting to create some kind of super-coffee, one wouldn't ever caffeinate it. One would only leave it unprocessed. It doesn't undergo caffeination.
If we applied the same construction to other terms, we'd get outright nonsense. I haven't had a heart attack; does that mean I'm fibrillated? Nor have I been beheaded recently; am I thereby capitated?
The Ngram graph for this looks interesting. It seems that caffeinated came in some time after decaffeinated, so presumably is a back formation. But bizarrely, caffeinated seems to have overtaken its more etymologically respectable cousin in the last few years! Why on earth would that be?
UPDATE: it's been pointed out in comments below that decaffeinated often gets abbreviated to decaf, and that if you combine these terms, decaf(feinated) is still winning the race.