Users may experience a Java error on Mac OS X if they are using Java version 1.3 and below. Please ensure your Java install is up to date.
NOTE! For gecko browsers (Mozilla, Firefox, Camino), the following may be the source of the error:

Mozilla will only use JRE 1.3.1, even if a later JRE is installed [Bug 197813]. To use a newer JRE, you will need to install the Java Embedding Plugin for Mac OS X. This plugin is beta quality software, and may have bugs and cause problems. Read the Readme before installing it.

Java for Windows, Mac OS, Solaris, and Linux can be downloaded here. Instructions for installation may also be found at that site.