Here's two that get me sometimes:
"rightfully"
When something happens *by right* this word is perfectly okay: "When the true heir was finally found, he rightfully took possession of the property from the impostor."
But when we're looking for a synonym for "correctly," the word to use is "rightly" and not "rightfully": "When she uses 'forte' to mean one of her strong points, she rightly pronounces it with just one syllable."
"backslash"
Man, I'm hearing this with increasing frequency. Most Internet URLs have one or more of these: /
That's a slash--just a plain slash.
THIS is a backslash: \
You don't ordinarily encounter a backslash in a URL; but when people are reading off a URL, say over the phone or in a radio ad, I keep on hearing them say "backslash" when they just mean a regular slash, and if you follow what they actually say, you don't get to the website.