Buying a stock just before its ex-date purely to collect the dividend, then selling shortly after. It usually does not work, and the arithmetic is the reason: the price typically drops by roughly the dividend on the ex-date, so the payout and the drop largely cancel. What does not cancel is the cost, since you pay commission twice, the 0.6% stock transaction tax on the sale, and 10% withholding on the dividend itself. It is listed here because it is a common beginner question, not because the numbers support it.
See every term in the full PSE investing glossary.