Here is a interesting bug I ran into today while using CF No Debug. If you use the <cftrace> tag with the _cf_nodebug parameter set to true, the tag will throw the following error:
Variable DEBUGGER is undefined.
In ColdFusion, the <cftrace> tag is implemented in CFML via the pre-complied WEB-INF/cftags/trace.cfm template. Now, this is simply a guess, but I bet that trace.cfm uses the IsDebugMode() function to determine if it should attempt to write trace info to the debugging query. Possibly something like the following:
<cfset factory = CreateObject("java","coldfusion.server.ServiceFactory")>
<cfset debugger = factory.getDebuggingService().getDebugger()>
<!--- add trace data to debugger --->
As I previously noted there is a bug with IsDebugMode() where it returns true when using _cf_nodebug = true, even though the debugging service is disabled. If something like the above code is used in trace.cfm, IsDebugMode() would return a false positive when _cf_nodebug is true and the ColdFusion ServiceFactory would return null for the debugger variable. When a null value is assigned to a ColdFusion variable it can lead to the undefined error we see in this case.
I've logged a bug with Adobe for this, but it is something to look out for if you use CF No Debug and start seeing "Variable DEBUGGER is undefined" errors.