As a matter of facts no one can forecasts when it can happen. But some people think this time will be different. Unfortunatetly bubbles are integral part of the system. Let's name the few important last one:
Oct 1987 - black monday
Sep 1992 - black wednsday
Aug 1998 - russian financial crisis and LTCM fail
Mar 2000 - dot com bubble
Sep 2007 - subprime loans
Apr 2010 - european sov. debt crisis
Almost all of them happened aftee a period of low voltality.
Current bubbles that could pop are: stock market, government debt, corporate debt, derivatives, even sudent loans. All of them on record high and cental bank easy policy is comming to an end.
Bubbles are not the end of the world, it only means a lot of traders are going to loose a lot of money. It is important to be careful.