They are rare in real life, but most of us have been in a situation like that one time or another.
I'll repost an example I posted to a similar question on reddit:
You are traveling in a foreign country and you oversleep your alarm clock. You only have 30 minutes to get to the airport or you will miss your flight home, and your sister's wedding, which is a few hours after your scheduled arrival. You have no cash on you, only 0.9 Bitcoins on your smartphone.
There is a shortage of taxis because of a major sporting event. For the same reason, all the ATMs are empty. Still, you are lucky enough to find one of the few taxis still vacant. However, the driver doesn't accept credit cards, only cash or bitcoin. He charges 1 BTC for the trip. He is in a grumpy, take-it-or-leave it kind of mood so bargaining won't work.
(1) You can either give up on the taxi, but then your chance of catching the flight is practically 0%.
(2) Or you can bet the 0.9 BTC on a quick round of Satoshidice, and that will give you a 90% chance of catching the flight, and not missing your sister's wedding.
Which option would you choose? 1 or 2?
The worst that can happen is that you lose 0.9 BTC and miss the flight. But isn't it worth taking a chance? It's your sister's wedding after all.
Having said that, I doubt that this accounts for most of the volume on satoshi dice. People are just being irrational.
tl;dr expected utility != expected monetary gain
ok, a supervening utility...though maybe a slight floor is that your more likely than not to still not get the flight, but I guess more than zero, so ok....good example