When AMD launched Phenom at lower-than-expected clock speeds, they told us that the 2.4 GHz was affected by an error that caused data corruption and instability.
As it turns out, ALL Phenoms are affected by this problem, and the only way to fix it is to update the microcode via a BIOS update. This update disables part of the L3 cache, and causes a 40-50% drop in L3 performance.
This translates to a 14% drop in real-world performance, with programs like Firefox seeing a 50% drop.
Also, it was discovered that AMD had sent overclocked processors to reviewers, which artificially made the Phenom look more competitive to Intel's Core 2 Quad. Some may see this as being an even bigger issue, as it makes AMD look insincere.