After upgrading Mountain Lion, you might find odd problems such as Safari crashing, or the system not showing the appropriate cursor at times, either in Safari or in other programs like Microsoft Office. When you might expect to see the standard arrow as the mouse pointer, the cursor will instead look like a crosshair or another alternate cursor.
This problem is not widespread by any means, and seems to be rooted around the user installing an incompatible Adobe PDF viewer Web browser plug-in.
Since Safari provides built-in PDF compatibility, for most people there has been no real need for an alternative PDF viewer plug-in. If you do need an alternative, though, then Adobe does offer a compatible plug-in with its latest Adobe Reader for OS X; however, those who have older versions of Adobe's CS suite or Acrobat Pro program may inadvertently install the plug-in when either repairing their current installation or reinstalling the program from scratch, steps that might commonly be done after upgrading OS X.
Users with such configurations have reported black browser screens when viewing PDF files, and in Mountain Lion have more recently seen odd cursor problems and browser crashes, and uninstalling the plug-in has resulted in these problems going away.
If you are experiencing odd problems with browser features or strange cursor behavior after upgrading to Mountain Lion, then you might try checking for the presence of the Adobe PDF-handling plug-ins called "AdobePDFViewNPAPI.plugin" and "AdobePDFView.plugin." These will be located in the /Macintosh HD/Library/Internet Plug-Ins/ folder and can be removed manually to uninstall them. You can then download the Adobe Reader plug-in from Adobe's Web site if you would like to use this instead of Apple's PDF engine.