Like Yay points out, it's a colloquial expression we use in Spain to say that something didn't go as expected but not in a dramatic way.
If we take the news from El País, we are reading that a very popular politician from Madrid is telling us in a distended way that she made a few bad choices. She's downplaying her mistake by saying "2 out of 500 ...".
It is an expression typically used in Spanish to dial down something that went wrong into something funny to explain or not important at all.
In a very formal context it could be inappropriate mainly if the mistake had really bad consequences. Like it's the case of this politician's mistake (but she keeps downplaying her responsibility in the situation)
Otherwise you can use it in any context. For example... dates, personal relationships, partners or any context where we made a bad choice.