For example, in conversation, in addition to foibles such as calling people by incorrect names, I frequently interpose words, unintentionally mix metaphors, conflate two terms for the same concept, and goof up common figures of speech. As a result, I inadvertently coin phrases.
I think I coined one that stuck, about 15-20 years ago. I remember very specifically that I was talking to a friend about college, since I was about to start undergrad. My good fortune was that since my mom was employed by the college in my home town, I got to attend tuition-free. When describing this to my friend, I said "It's like getting a full ride scholarship."
I don't recall that I'd ever heard the term before; maybe I had, but I remember that I later thought to myself that it didn't make much sense. Full ride? I think that I was combining "Fulbright" (a term that I knew related to academic grants) with "free ride" (a term used to refer to an all-expenses-paid venture) and came up with "full ride."
I've used the term several times since then, always hesitantly, and always because I really didn't know what the correct term was for the concept of a "tuition-free college education."
The bizarre thing is that I've heard (and seen) the term a lot since then, although almost always colloquially. However, it's been used as the title of a movie, and shows up all over the place to refer to the same thing.
Every time I see it, I think two things: 1) "That term makes no sense," and 2) "Did I make that up?" I can't really discern any etymology of the term based on the online links I can find.