> 1. Open text file
 >
 > 2.
 >
 > M-x longlines-mode
 >
 > M-x longlines-show-hard-newlines

At the time I'm here the buffer is already modified since modification
hooks are not inhibited in `longlines-show-hard-newlines', thus calling
`longlines-after-change-function' which subsequently causes to run
`longlines-post-command-function' which runs `longlines-show-region' a
second time _without_ saving the buffer-modified state though, ...

The attached patch should handle this.