instantly kill rift herald

16mins into game playing as olaf with a tracker's knife and blade of the ruin king I engage the rift herald with a lee sin only to find out I didn't need his assistance, on the engage I used olafs W just to find the rift herald with 7,000 odd health taking a insane amount of damage almost dying instantly(within a second of the engage). it was the only time I have experienced this bug and haven't tried to duplicate since.