By virtue of it happening, that's how I diagnosed it. Each time, OS came to a complete, absolute freeze, unrecoverable. Could not get the Force Quit menu to come up. Had to hard reset my MB Pro. Has happened several times.
Flash was not running on any of those hanging tabs, but honestly, does it matter? In what world is it acceptable for a process to freeze the entire OS?
Pray tell, how did you diagnose this? Was flash running on either of those pages?