emacs-lisp-mode, or latex-mode (AUCTeX)
Turn on longlines-mode.
Paste a long line (paragraph in the long-lines sense) into the buffer,
which should be wrapped correctly and displayed as a paragraph of text.
Select the entire paragraph and call `comment-region'.
Result: The comment mark (;; or % or whatever) is placed all over the
lines, and only the first line has it in the right place, i.e. at the
beginning.
(Also, when `longlines-wrap-follows-window-size' is turned on, then the
line/paragraph is reformatted.)
The expected (correct) behavior would be to add the comment command at
the beginning of the line in the file, i.e. at the beginning of the
paragraph, that is, after each hard return.
Note that the other expected thing there is that the syntax
highlighting (font-lock) works, i.e. that the whole comment is shown
using the right face. This, IMHO, is a second bug.