Misuse.

For language/usage nerds like me:  There is a wonderful thread at Volokh Conspiracy about confidently-misused words and phrases.  Here's the original bleg from Eugene:

I’m collecting examples of usage errors or usage controversies that bite people unexpectedly. In particular, what I have in mind is words or phrases that meet both of these criteria:

(1) You once — preferably after the end of your college education — confidently thought that the term, or a particular way of using it, was uncontroversially correct and effective.

(2) You later learned that many people think the term or the way of using it is wrong, or that the term also has a different meaning that makes it less effective at conveying the meaning you thought it conveyed.

Lots of examples provided by readers and not a few debates about whether an incorrect meaning has passed into correct usage by dint of frequency.  See enervate, for all intents and purposes, nonplussed, the lion's share, moot, spendthrift, etc.  

 I for one will confess that I have now been properly schooled with respect to the word "meretricious."

Comments

[MelanieB]

Oh fun. Thanks for pointing this out. I'm currently stumped but I'm sure I'll eventually think of some of my own bloopers.