This test puts the testing frame close to the left edge of a screen. If (as I usually configure) a vertical applications bar (the Launcher) is hidden, an icon of a started application is popping out for a second and then hides back. The test manages to make its first screenshot about the same moment. As a result, on the first screenshot there is a shadow of that icon, and comparison fails.

RULE "javax/swing/text/CSSBorder/6796710/bug6796710.java" Exception java.lang.RuntimeException: The test failed