basic idea
P(losing huge number of units) > 0.0
For example, we take one dice, you get 1-2-3-4-5 and I get 6. you roll and if you hit one of your numbers I pay you 1:1. If you hit my number, you pay me 1:1. Most likely I will go bankrupt. But on occasion, you will go broke rolling many 6's...
It isn't likely, but probability doesn't care. It can come up at any time, and if you are unlucky it can come up more often than it should. You might also play a bunch of sessions and win every one...
The more you play, the more the "unevenness" smoothes out and things are not as painful as they can be over a very short term experience.