This is not a duplicate of The "Flag comment" icon is too harsh because I actually like the image of the flag.
Rather, the problem is when you hover over it. On Meta, if you hover over the flag so it turns from white to black, the heights of the two images are not the same. It results in the control (and anything below it) being shifted up one pixel if the comment's length is short and does not increase the height.
For an example, see Should our FAQ contain a list of what's off topic and if so what should that list contain? and hover over the flag of a short comment and the flag of a long comment.
It's different on Programmers. On Programmers, the height and width are off. See Does tweeting 9 to 5 hurt job applicants? for an example. (Specifically, GrandMasterB's comment on the original question post)
Due to the way browsers render font sizes and line lengths, this may vary.
However, there is an extremely simple way to know something is wrong: the bottom of the hovered "flag" image is cut off.