If you can dream of a mistake possibly happening, it has already happened to someone, somewhere, sometime
The photo below is a shot of a portion of the front page of a newspaper. The partial headline relates to the story to the right and below. The gibberish on the left is a portion of the editor's notes to the writer of the featured story which is fully above the partial headline. I bet everyone is pointing fingers at everyone else as to how that editor memo made it into print.
This would be easy to do now. When i was a sports editor in the 60s, linotype operators cast each line (in reverse) from hot lead; then columns locked into place, inked, and a sheet of paper rolled for proofreading. Not that mistakes weren't made - it just happened slower.