The market looks calm. Under the surface, it's a very different story. Wednesday ended with modest gains: Market recap: • Dow Jones: +0.29% • $S&P 500(.SPX)$ : +0.38% • Nasdaq: +0.62% If you only looked at the major indices, you'd think it was another quiet day. But beneath the surface, one of Wall Street's most successful strategies is facing its biggest test in months. Momentum Investors Are Feeling the Pain For years, buying the market's strongest-performing stocks has been one of the most rewarding investment strategies. As long as winners kept winning, momentum worked brilliantly. Now, the tide may be turning. The Invesco S&P 500 Momentum ETF has fallen more than 7% in July, with several former market leaders, including Corning,