id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
1997	GUICtrlCreateDummy(), @error=1 on failure., trivial.	mvg		"@error support not mentioned in manual for GUICtrlCreateDummy()
(or any other GUICtrlCreate*() functions for that matter. (assumed after a quick browse))
So assuming its some leftover. (or possible backward compatible feature.)

Both 3.3.6.1 and 3.3.7.14
(Xp.32)"	Bug	closed		AutoIt	3.3.6.1	None	No Bug	GUICtrlCreateDummy