My favorite contranym: In court to “continue” something means to stop doing it and take it up later.
Firmament. It just sounds too heavy to be the sky.
Hasty sounds like pasty. It should mean slow.
Prude. Meretricious.
Yes!.. I mean no.
For me that word is “flurry” as in snow flurries. It sounds like it should be a lot of snow but it means very little snow. Maybe because the word is so close to “fury”? I don’t know.
Akimbo. I know if you’re standing arms akimbo, that means you’ve got your hands on your hips, your elbows out. But in my mind, it means your arms are flailing about.
Restive
I put rest in restive, instead of restless.
And it has something to do with a lady singing . . . right?
Whenever I used the word bemused used properly I end up, well, bemused, because long ago I learned the incorrect definition for the word and I can’t unlearn it.
That reminds me that for years I confused Atheist and Agnostic because I originally learned their definitions switched around.
“Slept fitfully”. My first thought is always of the opposite.
Whenever I see the word “vexed” I think it means to be confused or baffled by something, when actually it means to be angered by something.
Maybe I’m subconsciously confusing ‘vexed’ and ‘perplexed’?
Oh, I know! When I was a kid in church every week they sang that God “recalls his promises” and I knew what they meant, but I kept thinking of it like in a product recall – taking it back because it was defective.
“Salubrious” apparently means wholesome or health-giving, but it always sounds skeevy and skanky to me.
I only know the word because of a street in the town where I used to live called Salubrious Passage. Frankly it sounds like it should be the place where all the hookers hang out.
“I’ve got this terrible itch in my Salubrious Passage, doctor!”
You people obviously don’t know your high-end liqueurs. 
(And, in actuality, Chartreuse comes in two different colors: one yellow, and one green.)
There’s enough friggin words for shades of red, we don’t need another one.
Spendthrift
Privation
Dearth and boon.
I always want dearth to mean “too many,” and boon always feels like it should mean “burden.”
Befriend. It sounds like it’s the opposite of friend (befriend=defriend) and is too similar to betray.