If the leap second was really just expressed to the application as the 61st second of a minute, it wouldn't be so bad. But instead the second of 23:59:59 happen

If the leap second was really just expressed to the application as the 61st second of a minute, it wouldn't be so bad. But instead the second of 23:59:59 happens _twice_ (UNIX time steps backwards one second), which tends to confuse applications that assume time increases monotonously.