Last time I questioned whether "scrumptious" and "delectable" were really synonyms. I think the jury is still out. Regardless, there are other examples of words that are used as synonyms when they most definitely are not. And the worst culprit with these, I have found, are journalists, though misuse is quite common.
Here's an example: is the nation currently facing a "fiscal" crisis or a "financial" crisis? Or both? They are not the same, but you'd never know it from reading the newspaper, where the words are used synonymously. (The way policymakers talk about it doesn't help.)
There are many money words that are frequently used as sloppy synonyms, and with no background in economics I've been guilty of this myself on occasion (though at least I try). For example, what's the difference between "equity" and "capital"? (Not so straightforward, it seems, though they are not the same.)
Here's another: "science" and "engineering." Not the same, news reporters of the world. Scientists are not engineers and engineers are not scientists. I won't even get into "accuracy" vs. "precision."
And also, if I'm a scientist and I go through peer-review to publish the results of my research, I have not published an "article" in a "magazine." I have published a "paper" in a "journal." The Proceedings of the National Academy of Sciences is not a magazine like Vogue or Maxim. And there's a big difference between "A supposedly fun thing I'll never do again" or - God forbid - "Ten ways to drive your guy wild in bed" and "Metabolic cross-talk allows labeling of O-linked β-N-acetylglucosamine-modified proteins via the N-acetylgalactosamine salvage pathway."
But here are the two that I hate hate hate cannot abide!
1) Prison and jail. No, no, no, people are not sentenced to 30 years in jail. Nor, for that matter, three months in prison. If you know the difference between a misdemeanor and a felony (and no one uses those synonymously), then you know the difference between a prison and a jail.
2) Monkey and ape. This one drives me, pardon the phrase, apeshit. Humans are apes, as are chimpanzees, orangutans, gorillas, and gibbons. But apes are not monkeys and monkeys are not apes. Among a host of other differences, monkeys have tails.
Look, I don't walk around expecting people to know their prosimians (even though they are primates too), or to be able to elucidate the differences between perissodactyls and artiodactyls (or even sarcopterygians and actinopterygians), but considering how close to us on the evolutionary tree monkeys and other apes are, I think it is reasonable to feel the strong urge to go bananas (yes, that is deliberate) when people say that chimps are monkeys. You know a whale isn't a fish, you ought to know a baboon isn't an ape.
And when creationists say "I didn't descend from any monkey" I want to laugh and say "you are absolutely right!"
Then again, even a stopped clock is right twice a day.