Seriously, the exploit that it is possible to get loans interest free (or whatever that's now called) is known, it is not THAT hard to find out the time when this is calculated every hour and I also have been bitten by this every once in a while - it seems during a rally borrowers just take whatever is there, wait a bit until new lending offers pop up and then take these, cancelling the existing ones. Unfortunately Bitfinex does NOT seem to do interest calculation once an order completes over its lifetime or since the last midnight payment, whatever is shorter, but rather every hour on all active ones, for whatever weird reason.
While it is possible to exploit this, it doesn't seem to be done regularly. On a second thought, from the traders POV, it isn't worth the hassle. You can earn way more by effective trading, but you can also loose way more than the typical order of magnitude of the swap.
I'd guess that this unfortunate gap becomes largely visible only during flash crashes like the last days, where many people open positions during a very short timeframe. But there is the danger for an experienced and planful attacker to exploit this gap systematically, and probably the Bitfinex team is already aware of that.
Without knowing any internal details of the implementation, my guess is that it's non trivial to calculate the swap precisely in a way that scales out on a growing distributed system, since a complete trading position is comprised of N position changes multiplied with M involved loans.
