In current ZK org.zkoss.zel.Util, some exceptions are ignored and a stacktrace is output in the console directly.

Since these exceptions are used for debugging, they may cause confusion when parsing the server logs for errors.
example here:
https://github.com/zkoss/zk/blob/master/zel/src/org/zkoss/zel/Util.java#L542

Using the internal ZK logger instead of direct output to console would allow developers to change the log level for this class / package depending whether or not they want to see the trace in server logs.

For full ZK update: replace all direct .printStacktrace() in zk code by a logger call.
INFO seem to be a good log level for these calls (will allow for deactivation without removing warnings and errors)
https://www.zkoss.org/wiki/ZK_Developer's_Reference/Supporting_Utilities/Logger

Note: should be still visible by default if the developer doesn't manually suppress the logs.