I have this happen frequently. In my mind I know what I want to say, but what comes out is so very different and out of context.
I also find that I need to double check and triple check myself when getting directions for anything because, even though I hear what is correct, what my mind sees is something different. When it's really bad, I'll keep repeating the correct thing, but I still get it mixed up.
I find this usually happens when I'm in a depressive episode, in the midst of anxiety or when I'm stressed.
I feel like my brain is not wired right.