See main CR - but basically if the system time changes (eg via ntp adjustments) the timed-poll() that underlies Thread.sleep can return immediately, with no error indicator, causing the Thread.sleep to return early. This can cause problems with tests that use Thread.sleep to delay until other threads should be in the right state.
Correction: as per 6474541 it takes more than just a simple time change to trigger this bug. But this simple fix remains worthwhile.