It probably was an inside job to some extent. Some menial but smart guy was not really paid enough and said screw it, it worth the risk and gave himself a $5 Mill pay day.

It looks like they are going to have to multi sig everything.

Another thing I do when sending is to send a very small test amount to an address first just to see if it gets there. Once I see that hit, then I am pretty sure everything is ok and send the larger amount. Sure there is an overhead but hey you pay to minimize risk.