Opposite Day
I'm usually pretty careful (some might say anal) about the meanings of words. For instance, "uninterested" and "disinterested" mean very different things, and "peruse" means the opposite of what most people think it does.

So I was surprised recently to find that "ravel" can mean the same thing as "unravel". It is similar, in this way, to "bone"/"debone" ... but doesn't it seem like "ravel" should mean the opposite, to "tangle"? Turns out that that's right, as well. Ravel can assume either definition. It's an "autoantonym", a word that has two opposite meanings.

After some googling, I found this great list of autoantonyms. The best part is that you know many of these already; it's just that your brain is so good at inferring from context that it automatically decides on the contextually-correct meaning and the conflict is rarely apparent. My favorites:

• aught: anything / nothing
  
• custom: usual; normal / special; unique
  
• impregnable: impossible to enter / able to be impregnated
  
• resign: to quit; give up / to sign up again
  
• shank: latter part of a period of time / early part of a period of time