"It is inherently unsustainable and not really something that should be in operating cash flow," says Charles Mulford, an accounting professor at Georgia Institute of Technology and co-author of "The Financial Numbers Game." ------------------ --------------------------------------------------- --------------------Here's one fairly stunning example. Many investors probably think Yahoo! (YHOO:Nasdaq - commentary - research - Cramer's Take) generated a massive $1.56 billion in operating cash flow in the 12 months leading up to the end of September 2005. After all, that's what databases tallying the numbers reported by Yahoo! will reveal. In reality, 61% of this "operating" cash flow -- or $955 million -- came from the tax benefit Yahoo! got when employees cashed in stock options. That massive percentage means two things. First, Yahoo! is more likely to see operating cash flow take a hit as the new rules go into place. Second, it gives a misleading impression of how much cash Yahoo!'s core business is throwing off.