Potato, potato

Two words, very similar in appearance and pronunciation - just the presence or absence of an "i", which means an extra syllable, so you can easily tell the difference when you hear them. But the meaning is completely different. Yesterday, I heard two people who I would have thought would know better, use "depreciate" when they meant "deprecate".

And while I'm on the subject, "affect" and "effect". Similar in spelling and pronunciation, completely different meanings. I so often hear one when the speaker probably means the other.

And finally, "defuse" and "diffuse". So many people either think they mean the same thing, or don't know there's two completely different words, or don't know the meanings of both.

I'm not going to bore you with the very different meanings of these words, you can look them up.