In Css/JsImport.render(), the src/href attribute is temporarily removed:
// Temporarily remove href attribute while other attributes are rendered
// Restore href attribute
This cause incorrect rendering and ConcurrentModificationException when other threads rendering the same element. The sharing between threads is common for the CSS & JS elements added to a cached menu.
It is quite easy to reproduce: Just let jmeter hit localhost:8080/click-examples/home.htm about 40.000 times. After the test is finished use a browser to show the examples homepage. For me, the CSS and JS import elements related to menu is missing or rendered incorrectly.