As a fellow UB nitpicker, I'd like to hear more about your favorite UB triggers. E.g. in the space of file IO.

The C++ filesystem section contains the following, which basically means you can’t do safe IO when other programs are running in the same computer:

“ The behavior is undefined if the calls to functions in this library introduce a file system race, that is, when multiple threads, processes, or computers interleave access and modification to the same object in a file system.”

I'm not sure that means much, since that's true of literally every language that does file I/O.