Stocks flat to mixed as cliff talks continue

Stocks opened flat to mixed Thursday as negotiations continued in Washington over avoiding the tax increases and spending cuts of the "fiscal cliff."

On Wednesday, Apple stock dragged down the Nasdaq composite index.

Apple, once the stock market's darling, fell another 1.97% Thursday, It dropped 6.4% Wednesday to close at $538.79. The electronics maker is down 24% from its September high.

If you bought a technology ETF to diversify your tech holdings, though, you may have noticed that Apple's woes are yours as well. And that's because Apple is such a big slice of the largest technology ETFs.

Powershares QQQ, Qubes, as they're called, have 18.8% of their assets in Apple, according to Morningstar. QQQ fell 1.1% Wednesday.

Technology Sector Select SPDR has 19.2% of its assets in Apple. It fell 1.0% Wednesday.

Vanguard Information Technology ETF, 20.5% in Apple. It fell 1.0% Wednesday.

IShares Dow Jones U.S. Techology has 22.9% in Apple. The fund fell 1.3% Wednesday.

These funds' huge positions in Apple point to a problem with indexing in a stock sector, especially when the index gives greater weight to stocks with larger market capitalization - stock price multiplied by shares outstanding.

Even after Wednesday, Apple is a behemoth, weighing in at $506.8 billion. Rival Microsoft, the second-largest holding in QQQ, has a market cap of $224.5 billion. The tech sector is so top-heavy by cap weighting that the five largest members of the Nasdaq 100 index are 41.5% of the index. But if you buy a tech ETF, your biggest slice by far is Apple.

USA Today