If you create a new file while
cp is operating, it's likely that it won't be picked up. That might depend on the
cp implementation: some gather a list of files when they start, others do it by chunks. If it's a recursive copy, all the
cp implementations I've seen work directory by directory, so if you add the file to a directory that
cp hasn't traversed yet, it will be copied.
If you add a file midway and you want to copy it, run
rsync afterwards (after the copy is finished and the new file is fully written). Rsync will only copy the added file.
You can run rsync from the start (use
rsync -a to do a recursive copy and preserve metadata). Rsync can do pretty much everything
cp can do and much more, so you can ignore
cp and always use
rsync if you like (except on embedded systems that don't have