File RAMJobStore.java, class TriggerComparator, method compare():
    public int compare(Object obj1, Object obj2) {
        TriggerWrapper trig1 = (TriggerWrapper) obj1;
        TriggerWrapper trig2 = (TriggerWrapper) obj2;

int comp = trig1.trigger.compareTo(trig2.trigger);

if (comp == 0)
                return trig1.trigger.hashCode() - trig2.trigger.hashCode();

return comp;
    }

The problem is with this line: return trig1.trigger.hashCode() - trig2.trigger.hashCode();

Hash code may generate exteme numbers, basically you could run in a situation like:
Integer.MIN_VALUE - Integer.MAX_VALUE
per comparator contract this should be a negative number, however due to overflow it might not be.

This problem completely breaks Quartz functionality when it occurs.

The easy fix is to replace the line with: return trig1.trigger.getFullName().compareTo(trig2.trigger.getFullName());


Illustrative example of the problem:
TreeSet set = new TreeSet(new TriggerComparator());

for (int n = 1; n < 7; n++)
{
CronTrigger cr = new CronTrigger("Trigger1", n + "MemGroup");
cr.setCronExpression("00 00 16 * * ?");
TriggerWrapper tw = new TriggerWrapper(cr);
set.add(tw);

}

for (int n = 0; n < 7; n++)
{
CronTrigger cr = new CronTrigger("Trigger1", n + "MemGroup");
cr.setCronExpression("00 00 16 * * ?");
TriggerWrapper tw = new TriggerWrapper(cr);
set.remove(tw);
}

After this code runs -- TreeSet is not empty due to comparator problems.

Another example: see attached configuration files. With them only two tasks are ever run.