The main repo I work on for my job is open-source, and yes, it happens fairly regularly that someone opens a nonsensical PR that might randomly perturb some Markdown or YAML file, or attempt to merge commits from some other developer's in-progress branch.

I'd say we only get them about once every week or so, but then the repo is not anywhere near as high-profile as the Unreal Engine, nor as likely to be on the radar of children.