I had to search for this one... I couldn't recall that I had written this error message, and I hadn't
This is an error message that java generates when the class is corrupt or not completely downloaded.
Look at this answer I borrowed from the java forums:
The "bad magic number" indicates that the .class file is either corrupt or your server is sending some other data stream than the class format. The first bytes of a class file should be the magic number CA FE BA BE in hexadecimal.
Applets can be loaded from any HTML source as long as the applet .class, .jar or .cab file is correctly reurned to the browser.
This means that your YawApplet.class file probably is not complete or that there occurs some transmission errors.
The first thing to do now, is to replace the .class file and see if this does the trick. Either reinstall Yawcam or just download the applet from here: http://www.yawcam.com/newapplet/YawApplet.class
The .class file should be located in yawcam's stream directory (i.e. c:\program files\yawcam\stream)