I can't recall the exact wording of it, but I believe the rule on drowning in the core rulebook states that if someone who is drowning fails their save, they go to 0 health, and then on down into the negatives on subsequent saves. Some people choose to interpret this literally, claiming that failing their first save against drowning should put them to 0 even if they were lower than that previously, effectively healing them back up to 0.

Yes, it really is that stupid.