If you have downloaded latest FlexUnit 4.1 recently and run “turnkey” test project, you will encounter a strange error:

“Error #1053: Illegal override of NullDescription in org.hamcrest.NullDescription”

After further investigation, I found that they use the same hamcrest component built that comes with Flash Builder 4 (hamcrest-1.0.2.swc)…, you will have to download latest version from http://flexunit.digitalprimates.net:8080/view/Supplemental/ in order to work properly. Hope it helps!