The other possibility is that the web server is sending the file to clients as text rather than binary data. Make sure the web server is configured to send files that end in ".class" with a MIME type of application/octet-stream. Many web servers send files as type text/plain which often works but causes problems on a few servers. In particular, WebStar needs to change the action to "binary" and the MIME type to "application/octet-stream".
It's also possible on some platforms that Netscape just can't find the .class file; that is, it isn't in the directory where Netscape is looking for it. Technically, this isn't really a ClassFormatError, but this is how Netscape reports it on some platforms and versions.
Posted in: Java