XML - WstxParsingException: Illegal processing instruction target ("xml")

I got the following error when I was restarting JBoss AS 7.

com.ctc.wstx.exc.WstxParsingException: Illegal processing instruction target ("xml"); xml (case insensitive) is reserved by the specs.

JBoss complained it could not parse the standalone.xml.

Soon I realized that I accidentally added a empty line before the xml declaration. Something like:

[empty line]
<?xml version="1.0" encoding="utf-8"?>
[other xml content]

So please keep in mind whenever you encounter this error, do check your xml file and put the xml declaration as the first line of the file.