In January 2005, someone using the name "Rahodeb" went online to a Yahoo stock-market forum and posted this opinion: No company would want to buy Wild Oats Markets Inc., a natural-foods grocer, at its price then of about $8 a share.

"Would Whole Foods buy OATS?" Rahodeb asked, using Wild Oats' stock symbol. "Almost surely not at current prices. What would they gain? OATS locations are too small." Rahodeb speculated that Wild Oats eventually would be sold after sliding into bankruptcy or when its stock fell below $5. A month...