For example:

SEVERE: javafx.scene.control.Control loadSkinClass Failed to load skin 'net.jonathangiles.javafx.control.skin.ComboBoxSkin' for control ComboBox@35175422[styleClass=combo-box]
java.lang.ClassNotFoundException: net/jonathangiles/javafx/control/skin/ComboBoxSkin
 at java.lang.Class.forName0(Native Method)
 at java.lang.Class.forName(Class.java:169)
 at javafx.scene.control.Control.loadSkinClass(Control.java:878)
 at javafx.scene.control.Control.impl_cssSet(Control.java:974)
 ...