Using quartz 2.3.1 I get the following exception occasionally on startup of a webapp in Tomcat.

java.util.ConcurrentModificationException
        at java.util.Hashtable$Enumerator.next(Hashtable.java:1387)
        at java.util.Hashtable.putAll(Hashtable.java:523)
        at org.quartz.impl.StdSchedulerFactory.overrideWithSysProps(StdSchedulerFactory.java:495)
        at org.quartz.impl.StdSchedulerFactory.initialize(StdSchedulerFactory.java:472)
        at org.quartz.impl.StdSchedulerFactory.getScheduler(StdSchedulerFactory.java:1552)

This is due to StdSchedulerFactory.overrideWithSysProps which looks like this:

By calling putAll the entrySet() iterator is used on System.getProperties() which is not a thread-safe way of traversing System properties.

The fix would be to instead traverse the System properties by using the Enumeration propertyNames() and calling getProperty on each property name.

Happy to provide PR.

CC @SuperEventSteven