To reproduce:
-add an AnchorPane and a Button
-move button to the left border of Content Panel and get alignment guide for the button and anchorPane
-move the button horizontally to the left until left panel of SB -- yellow image for button's "ghost" is still drawn --BUG
-move mouse pointer to newly added anchorPane in Hierarchy -- now alignment guides are confusing: if user drop the button it will appear at another placed not marked by guides -- BUG