longlines.el has a bug you can reproduce with Emacs -Q as follows:

(1) Visit an arbitrary file, for example, longlines.el

(2) M-x longlines-mode

(3) M-x longlines-show-hard-newlines

The buffer is considered modified.  The reason for this bug is bad
handling of `buffer-modified-p' and `inhibit-modification-hooks' in the
associated functions.  The attached patch should cure this.  If no one
objects I'll install in a couple of days.