When word gets out that Buffett has bought a particular stock, its shares jump immediately. After all, if the Oracle of Omaha wants into a company, surely it has a bright future, right? Unfortunately, determining whether Buffett has actually bought a company is not so easy.
Sure, as described here, we can follow the mandatory Berkshire filings which detail its ownership in various securities (and many copy-cat investors do so diligently), but Buffett is not the only portfolio manager at Berkshire. But don't take my word for it, as this statement comes from Buffett himself:
"The media continue to report that "Buffett buys" this or that stock. Statements like these are almost always based on filings Berkshire makes with the SEC and are therefore wrong. As I’ve said before, the stories should say "Berkshire buys."...Even then, it is typically not I who make the buying decisions. Lou Simpson manages about $2½ billion of equities that are held by GEICO, and it is his transactions that Berkshire is usually reporting."
As an example, shares in CarMax rose 7.5% on the day a filing from Berkshire indicated it had taken a position. Presumably investors assumed Buffett had seen something in the valuation that the market hadn't. However, Alex Taylor refutes the fact that KMX was a Buffett purchase in a well-written article here.
So even though you know how to access Berkshire's filings, you may not know the buyer behind the buys. Your best defense is to do your own homework and to know yourself what you're buying.