Great performance can be a big warning sign, according to Vanguard Group founder John Bogle. "People hawking a particular fund have no idea how long it will continue to do well," he is quoted as saying. "They'll say, 'Our fund went up 500% in the last 10 years, and the index fund only went up 320%, so indexes are overrated.' That's usually the end for that fund." Read the original story from MarketWatch.