Happened yet? Has it ever
Before the market crashed in 2000, something fundamental changed. Though the overall indexes (like the Dow and the S&P 500) kept going up, those rises were being fueled by only a handful of companies, mostly Internet-related ones.
The lesson: Sizable increases in just a few stocks can mask what's going on in the broader market and signal that a bull is nearing a top, says Larry Haverty, a portfolio manager with Gamco Investors, an institutional investment firm. So keep an eye on market breadth - that is, how many stocks are rising compared with how many are falling.
If more stocks on the New York Stock Exchange are hitting new 52-week lows than new 52-week highs, that's a bad sign.
In mid-July nearly twice as many stocks were hitting new highs as new lows - a comforting sign - but by month's end this stat had turned downright ugly.