Dysfunctional Rules IX: 1d3 Dysfunctions from the 8th Level List

[Malimar]

In Pathfinder, when a creature with the Ferocity special ability goes below 0 HP, with zero points of nonlethal damage marked, it will fall unconscious because its nonlethal damage total, which in this case is zero, is higher than its hotpoint total, which would be a negative number. This is because Ferocity only prevents you from going unconscious due to your hit point total going below zero, but it doesn't do anything to stop nonlethal damage at all.

I think the same is true of the Diehard feat (in both editions), which has probably been covered before.