I was experimenting with something on the just released WildFly 11 and lazily added the "system-properties" element within the server element, after the socket-binding-group in standalone.xml. Something like this:

<server xmlns="urn:jboss:domain:5.0">
    .....
    <socket-binding-group>
    ...
    ....
    </socket-binding-group>

    <system-properties>
        <property name="test.wfly" value="hello world"/>
    </system-properties>

</server>

Then booted the standalone server and ran into this improved error message at startup:

17:51:32,950 ERROR [org.jboss.as.controller] (Controller Boot Thread)

OPVDX001: Validation error in standalone.xml -----------------------------------
|
|  510:     </socket-binding-group>
|  511:
|  512:     <system-properties>
|           ^^^^ 'system-properties' isn't an allowed element here
|               
|                Elements allowed here are:
|                  deployment-overlays   management            system-properties    
|                  deployments           paths                 vault                
|                  extensions            profile              
|                  interfaces            socket-binding-group 
|
|  513:         <property name="test.wfly" value="hello world"/>
|  514:     </system-properties>
|  515:
|
| 'system-properties' is allowed in elements:
| - server
| "
|
| The primary underlying error message was:
| > ParseError at [row,col]:[512,5]
| > Message: WFLYCTL0198: Unexpected element
| >   '{urn:jboss:domain:5.0}system-properties' encountered
|

I understand why it errored out (it expects the system-properties to be after the "extensions" element, than at the end) and I fixed the config to get past it. However, the interesting bit is that the error message that it threw states that "system-properties" isn't allowed at that location and the elements that are allowed there displays a bunch of elements which also includes "system-properties".

I haven't looked up the code to see how that log message is generated and if it's just a logging issue or something more. Thought of noting it here in the list.

-Jaikiran