I introduced a similar bug into one of my products in the past (Be honest, who hasn't?). But I'm surprised here because Klarna is a quite mature product and something like this shouldn't really happen at that stage.
Oh, it can definitely happen even in mature products. One I worked on had pretty much the same issue as Klarna (people seeing others' info) when someone updated a web client library we were using to a new version that subtly changed how it handled concurrency.