If MessageBox was used correct, it should give error 0.
But now, the error code is the same as the return value.
Code:
If NOT @LOGONMODE
	Break ON
EndIf
Dim $SO
$SO = SetOption("Explicit","On")
$SO = SetOption("NoMacrosInStrings","On")
$SO = SetOption("NoVarsInStrings","On")
$SO = SetOption("WrapAtEOL","On")

$SO = MessageBox("Hello World","My First KiX Script")
? "Return Value: " + $SO
? "Error " + @ERROR + ": " + @SERROR
? "Kix Version: " + @KIX
Shows on screen:
Code:
Return Value: 1
Error 1: Onjuiste functie.
Kix Version: 4.53 Release Candidate 1