Spotted in our nightly headful test build:

TooltipTest > testChangeShowDelayCssTooltip() FAILED
    java.lang.AssertionError: number of TextRuns in a TextLine cannot be less than one: 0
        at javafx.graphics@24-ea/com.sun.javafx.text.PrismTextLayout.createLine(PrismTextLayout.java:936)

The assertion error is not in the test, but rather due to an "assert" statement in the runtime. This suggests a product bug rather than a test bug.

This is the only time I can remember seeing such a failure.