Tomcat: Starting in Debug mode causes memory leaks?!

When I
* _Run_ my web application in Tomcat 6 from IDEA (with "Deploy applications configured in Tomcat instance" checked)
* then undeploy (from IDEAs "Server" tab),
* then open the Tomcat manager and click on "Find Leaks" button
then Tomcat says "no memory leaks detected".

When I do the same, but use a _Debug_ configuration, Tomcat says

   The following web applications were stopped (reloaded, undeployed), but their
   classes from previous runs are still loaded in memory, thus causing a memory
   leak (use a profiler to confirm):
   /fit4extjs4

How comes?
Having a debugger attached should not prevent unloading, right?