In some cases, the debugger stops on an Exception Breakpoint, even if it's Condition is set to 'false' statement.
In the debugger output, there is something like:

Exception breakpoint hit in java.lang.ClassLoader at line ? by thread main.
***
null
***

Thread main stopped at ClassLoader.java.findBootstrapClass.

I was able to reproduce this on two different systems:
Product Version: NetBeans IDE 6.5 RC2 (Build 200810270001)
Java: 1.6.0_07; Java HotSpot(TM) Client VM 10.0-b23
System: Linux version 2.6.24-19-generic running on i386; UTF-8; en_US (nb)

and

Product Version: NetBeans IDE 6.5 RC2 (Build 200810270001)
Java: 1.6.0_10-rc; Java HotSpot(TM) Client VM 11.0-b15
System: Windows XP version 5.1 running on x86; Cp1250; cs_CZ (nb)

Steps to reproduce:
1, open "MemoryView" project - http://qa.netbeans.org/modules/debugger/promo-h/MemoryView.zip
2, with the MemoryView.java open, create new Exception breakpoint, as the Exception Class Name enter:
java.lang.ClassNotFoundException  and for Condition type: false
3, debug the project
-> the situation described above should occur

Another approach (which I was able to reproduce only on the system with Ubuntu)
1, create new simple J2SE project
2, modify Main class, so in the main method looks like:
    public static void main(String[] args) {
        Main m = new Main();
    }
3, create the Exception breakpoint the same way like in previous case and try to debug the project.