Opera has a bug related to its handling of webfonts (fonts that are embedded in Web pages that you don't have to install yourself): if too many webfonts have been cached (about 25 in my experience), Opera will sometimes show the wrong webfonts on a page that uses them. This would normally not be a problem if many pages didn't use webfonts for special symbols and icons; and if Opera wrongly chooses such a font, the entire page becomes unreadable. This bug occurs on all operating systems.
Note: clearing the cache by checking "Delete entire cache" in the "Delete Private Data" window does not delete webfonts; this is why you have to do the above procedure manually.
You can use a program (for example CCleaner), batch file, or script to automate the procedure.
If having to delete the webfont cache periodically annoys you, you can do one of the following: