Hi, Alan.
I never used a Mac, so I'm only guessing. But this problem happens often in Windows versions of Quicken because of rounding. Quicken often retains 6 or more decimal places internally, even when showing us only 3 places. So a
4-for-3 split on stock when I hold 200 shares would entitle me to 66 2/3 new shares. Q might show that I now have 266.667 shares, but secretly it knows that I have only 266.6666667. So if I try to sell all 266.667, it will accuse me of trying to sell .000333 more shares than I have. If the sale somehow gets recorded anyhow, Q will show me that I own zero shares, but internally it will be remembering that missing .000333 shares.
Some variation on this little example is what you may be running up against.
The usual fix is to go back through all the transactions in that stock, chronologically. Be sure that the arithmetic on each transaction is done in such a way that the number of shares that Q shows onscreen is the same as the number calculated internally. On reinvested dividends, for example, always enter the total cost in dollars and even pennies and the shares in even thousands (.000). Let Q calculate the price per share, but don't ever let it calculate the number of shares.
If that's not it, post back. Some Mac/Quicken gurus hang out here and can probably help.
RC