Understanding risks and probabilities
I was just talking to a former co-worker who had just returned from Las Vegas. Although I've been to Las Vegas a few times, I've never gambled while I was there. The expected value of gambling is negative, after all, and I really don't think that I'd find gambling entertaining enough to justify calling any gambling losses an "entertainment expense." My former co-worker, on the other hand, always gambles when he's in Las Vegas.
When I explained my justification for not gambling – the fact that's the best you can do is to lose money in the long run – my former co-worker said, "But that's only if you play the best odds."
"And if you don't do that," I said, "you'll lose even more, won't you?"
Perhaps it's a good thing that this particular former co-worker doesn't work in information security or risk management any more.