This has happened to me one-too-many times, so I figure I’d post it:
I’m writing a longish comment in some code. I write it on a single line at first, then call fill-paragraph to let Emacs break it up into multiple lines of sane width. But, it prefixes each line with some text I’ve recently killed. The same prefix pops up when I call open-line as well.
I dug into this a bit, and found that I must have accidentally set the variable fill-prefix (which is bound to C-x .). To undo this, just call set-variable and set fill-prefix to nil, and go back to happily opening lines and filling paragraphs.