I'm using a type 4 (pure Java) JDBC driver in an applet. It works fine in Netscape, but doesn't work properly in Internet Explorer. Why not?

Microsoft's VM loads classes/drivers differently than the Java VM in Netscape browsers (and Sun's reference implementation). Just having a Class.forName(driverClassName) line is insufficient, as it doesn't consider it an active use. You'll also need to create a new instance of the driver Class.forName(driverClassName).newInstance() which registers the driver with the driver manager (java.sql.DriverManager.registerDriver(new DriverClass())).