Step 1. Request transfer of available funds from brokerage to external checking account.
Step 2. Sell stock.
Step 3. Put in a buy order, for less than the proceeds of the stock sale.
Result: E*TRADE sets aside settled funds for the buy order, even though they had been earmarked for transfer. So the transfer fails. (If the funds had already been transferred, E*TRADE would be perfectly happy to use unsettled funds from the stock sale.)