tomcat shutdown error

I use tomcat 6.0.32. When I do shutdown tomcat says that

tomcat did not stop in time. pid file was not removed

I get some memory leak error messages ..while doing shutdown . The error message are like below.

Sep 5, 2011 3:49:41 PM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads SEVERE: The web application [/abc] appears to have started a thread named [failedAuthRateLimit_Rate开发者_开发百科LimitAger)] but has failed to stop it. This is very likely to create a memory leak.

What could be the reason ...does the memory leak prevents tomcat from shutting down ?

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The error message doesn't prevent the tomcat to shutdown, but it warns you that there is a potential memory leak which should be fixed.

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There were some worker threads , which were preventing proper shutdown of tomcat. Now I resolved it, by properly shutting down the worker threads before shutting down the tomcat