There is a reason why groupthink is often the most dangerous concept in the financial industry. When it works: it makes everybody richer. When it fails, it results in the premature end of many asset managers. Nowhere is this better seen today than in the top holders of Apple stock, which according to CapIQ, is held by 1,933 institutions that have at least one share holding in the name, and 402, which have at least 100,000 shares. Cross-referencing the top 100 holders of AAPL stock with the top 100 holders of a recent tech casualty, Cisco, shows that the of the top 100 Apple holders (one of them being Steve Jobs), 93 are also present in Cisco. And a result of the past two-days' drubbing in these two names, these massively cross-correlated top holders in the two stocks have seen over $18 billion worth of P&L losses in the past two days alone. Now many will respond that these firms have also experienced massive profits on the upside, which of course would be correct. However, what many will also conveniently ignore, is that these very firms will all too often leverage unrealized profits and use these as margin to purchase additional stocks, in essence re-creating a new cost basis with every single remargining of the stock. Therefore, should there be a sell of in Cisco, or heaven forbid, Apple, it will pull the rug from the entire market. It also explains why mysterious buying will often materialize: the last thing the market can afford is a massive domino-style sell off due to the plunge in one single name. So far today, Apple dodged the bullet. Cisco still has not. At some point these mutual and hedge funds, however, will receive margin calls to fund over $18 billion in cash collateral, especially since mutual free fund cash is currently at all time lows just over 3%. When that happens, not even Brian Sack will save the market.