Correct. I have seen that happen and on investigating further, it is caused by the flyspell package if the string is detected to have a typo. As seen in the gifv, those strings are gibberish and so flyspell puts overlays on those.. and maybe some weird regexp match is causing the same overlay to be put on the whole block?
(defun make-flyspell-overlay (beg end face mouse-face)
"Allocate an overlay to highlight an incorrect word.
BEG and END specify the range in the buffer of that word.
FACE and MOUSE-FACE specify the `face' and `mouse-face' properties
for the overlay."
(let ((overlay (make-overlay beg end nil t nil)))
(overlay-put overlay 'face face)
(overlay-put overlay 'mouse-face mouse-face)
(overlay-put overlay 'flyspell-overlay t)
(overlay-put overlay 'evaporate t)
(overlay-put overlay 'help-echo "mouse-2: correct word at point")
(overlay-put overlay 'keymap flyspell-mouse-map)
(when (eq face 'flyspell-incorrect)
(and (stringp flyspell-before-incorrect-word-string)
(overlay-put overlay 'before-string
(and (stringp flyspell-after-incorrect-word-string)
(overlay-put overlay 'after-string
Next time, this happen I will put debug statements in the flyspell-highlight-incorrect-region function.
I expected that, but did not investigate what caused that as it looked right. Now I know that probably flyspell is causing this..