I suspect you may be thinking of the thing where people prefer e.g. a (A1) 100% chance of winning 100€ (how do I make a dollar sign?) to a (A2) 99% chance of winning 105€, but at the same time prefer (B2) a 66% chance of winning 105€ to (B1) a 67% chance of winning 100€. This is indeed irrational, because it means you can be exploited. But depending on your utility function, it is not necessarily irrational to prefer both A1 to A2 and B1 to B2.
I suspect you may be thinking of the thing where people prefer e.g. a (A1) 100% chance of winning 100€ (how do I make a dollar sign?) to a (A2) 99% chance of winning 105€, but at the same time prefer (B2) a 66% chance of winning 105€ to (B1) a 67% chance of winning 100€. This is indeed irrational, because it means you can be exploited. But depending on your utility function, it is not necessarily irrational to prefer both A1 to A2 and B1 to B2.