Here is my problem on SO :
Any help welcome !
Ironic, considering the actual problem is a StackOverflow:
You can use the workaround mentioned in the bug, or this behavior is fixed in FlexUnit 4.1 beta, which can be found here:
A release version of 4.1 is coming very shortly. The problem is your machine is fast enough to get one extra test in before the framework forces it to wait until the next frame. Flash Player throws a Stack Overflow when that stack depth gets to deep and it is being caught in a try catch in an inopportune place, and subsequently tracing out the 'Whoa' statement.
In FlexUnit 4.x originally, we used a counter and an approximate frame length for this, which, on very rare occasions linked more to the machine than the tests, could allow this to occur. In 4.1 we actually monitor the frames as they go by which should permanently eliminate this problem.
Tank you very much !