The heavy check mark icon ( ; hex: / dec: ) displayed wrongly / differently than other browsers.
Several month ago in an update the check mark icon got a black border in Opera and looks bad. You can see it in this page https://stackoverflow.com/a/18294705, please open it and compare it in different browsers.
When I copy this icon it looks normally, but when I paste it looks like in the bottom of the picture, the checkmark with green color and black border.