looking at the code from 4.1.1 in org.apache.activemq.thread.Scheduler, in the cancel method:
the Runnable task argument is passed to clockDaemon.remove(). i think this is incorrect. ScheduledFuture ticket should be passed to clockDaemon.remove().
the javadocs of ScheduledThreadPoolExecutor.remove discuss the possibility that Runnables might be stored in some other form internally, so calling remove with a plain Runnable might not do anything. I think the solution is to call remove with a ScheduledFuture, which is how they are stored internally in ScheduledThreadPoolExecutor.
i came across this bug after upgrading to the java 5 version of backport-util-concurrent 3.0. that version makes more assumptions about the types that are passed into ScheduledThreadPoolExecutor.remove, so when you pass in a regular Runnable you'll get a ClassCastException.
this is trivial to fix, so i don't think a patch is necessary. also i think this might address the memory leak mentioned in