A common mistake that I do over and over again is that I put certain code into the skin class of a custom control while it should have been inside the control class itself.
I just noticed this again when going through the list of bugs filed for the PopOver control of ControlsFX. I contributed this control, hence I feel responsible for fixing those bugs (when time permits). One of the issues mentioned that it is hard or impossible to change the styling of the PopOver control. After some investigation I realised that I am adding a stylesheet to the root pane of the PopOver inside the skin class. This is working but it causes a problem when applications have already added their stylesheets when the PopOver is created. The PopOver stylesheet will then override the application’s stylesheet and only because it is added too late.
The only way to actually style a PopOver is to listen to the skin property and wait for the skin to be set. Once set a stylesheet can be added to its children. Not a very elegant solution.
In case of the PopOver control I do have a very good excuse for adding the stylesheet inside the skin class. PopOver extends PopupControl and you can not