Certain methods of certain swing components can only be called on the event-dispatching thread. The problem is is that it is almost impossible to determine which methods of which components this applies to.

An example is JEditorPane.setpage(), this method creates a new thread in order to handle the loading of a page. This is very thread unsafe and has to be run on the event dispatch thead, wither through an event handler or by using SwingUtilities.invokeLater(Runnable) or SwingUtilities.invokeAndWait(Runnable)

This is very badly documented and this should be indicated in the API docs