Going back one second is what generally happens now when a Linux box does the "inserting leap second" thing. It goes from 23:59:59.999999 to 23:59:59.000000, then runs that whole second again. You get to 23:59:59.999999 again, and then you finally roll over to 00:00:00.000000.
From the perspective of the typical time_t rendering of Unix time, there is no way to uniquely represent that 61st second. It just "disappears".
Having lived through systems going backwards some 17 seconds due to a botched NTP-GPS appliance config, I can tell you that what died at that particular site was all of the locking code that used wall time clocks instead of monotonic clocks. They all CHECKed and died when their preconditions were no longer valid.
This didn't have to happen. They could have used monotonic from the get-go, and then they wouldn't have died when the clocks got yanked backwards 17 seconds to the proper time base.