Actually, I did (~~cotton on to~~) ... imagine ... a way to exploit credit-card roll-overs:
Mr X has a credit card in Dudes with $0 owing, $10,000 limit.
Mr X. gets an offer to take up a credit card in Sorcerersbank Inc, with a 6-month rollover @ 2%.
Mr X opens a betting account at Vampires on-line, and places $9,000 in, via Dudes credit-card B-Pay. No transaction fee, 30-day interest free.
Mr X applies for new credit card in SorcerersBank, rolls over the $9,000 now owing in Dudes, and therefore owes SorcerersBank $9,000 @ 2% for 6 months, and owes $0 to Dudes.
Mr X gets a new SorcerersBank credit card of $12,000 limit with $9,000 deficit, but Mr X NEVER uses the new credit card for any retail purpose, just makes the regular monthly minimum payments.
Mr X decides not to bet on the ponies after all and transfers his $9,000 balance in the Vampires on-line betting to his savings account.
Mr X deducts $9,000 from his savings account, invests in Infallible Pty Ltd, and 6 months later has .... (let's imagine $8,000 - this is a fairy story after all).
Mr X has paid no transaction fees, no credit card interest other than the 2% on the rollover offer. Mr X has borowed $9,000 for 6 months @ 2%.
Mr X gets a phone call from Vampires on-line "please explain ..." Mr X explains that he has seen the light and joined Gamblers Anonymous, just in the nick of time, and therefore never actually made any bets.
Treat this little tale as you will. Always read the fine print.