This time it's words.
There are not many words/phrases I don't know the true spelling/usage/meaning or even origin for, but recently I've been using the subtitles on telly a lot, and now I'm thinking there's MASSES I don't know! I never realised I missed so much in the conversation! (I'm more a reader than a watcher.)
The latest was 'derring-do'. I always thought that it was daring do. As you would, logically, because it refers to someone doing something daring...
Another one from a few years ago was hiatus hernia. DH had one when he was a baby and I got him to say it slowly out loud for me several times so I knew what he was talking about. But all I could hear was higher titernia. 
It was a while before I found out what the actual words were! (I blame DH and his lack of separating the words properly, or indeed just writing it down for me 
.)