It drives me up a tree when people get idioms wrong. (This counts for this thread since a bunch of the time it comes down to an issue of spelling online.)
My mom says "checker box" instead of "chockablock"
My sister says "For all intensive purposes," not "For all intents and purposes"
When people “I could care less” when they mean they couldn't care less.
When people try and "make ends meat" when they should be trying to "make ends meet."
When people say their interests have been "peeked" or "peaked" not "piqued."
It's "scot free" not "Scott Free."
It's "bated breath" not "baited breath"
It's "should have" not "should of" - though this one can conceivably be shortened to "should've" and sound like the latter, so...
Edward hires Vivian to be his "beck and call" girl in Pretty Woman, not his "beckon call" girl.
Saying something is a "mute point" when it's actually a "moot point" though there is something to be said for a "moo point."
My mom says "checker box" instead of "chockablock"
My sister says "For all intensive purposes," not "For all intents and purposes"
When people “I could care less” when they mean they couldn't care less.
When people try and "make ends meat" when they should be trying to "make ends meet."
When people say their interests have been "peeked" or "peaked" not "piqued."
It's "scot free" not "Scott Free."
It's "bated breath" not "baited breath"
It's "should have" not "should of" - though this one can conceivably be shortened to "should've" and sound like the latter, so...
Edward hires Vivian to be his "beck and call" girl in Pretty Woman, not his "beckon call" girl.
Saying something is a "mute point" when it's actually a "moot point" though there is something to be said for a "moo point."

Teenaged X-Files obsession + Bermuda Triangle episode + Self-led school research project = Atheist.