Annoying Video Game Helper: The Grimrog's Voodoo EX ability can cause this onto you. If it hits you, it applies Dark Voodoo. This turns healing done on you into damage against you for a short duration up to 170 damage.
Game Breaker: The balance of the game went many places in development, but the ultimate for the Psychopomp bloodline which doubled their main sources of damage and healing while reducing the duration of negative effects on them for 8 whole seconds was considered to be an issue until a reduction in the total net value of said damage and healing increase happened. The "X-Strike" ability of Nomads' was considered to be pretty bad as well, as being hit by it guarantees you to be stunned and take a chunk of damage while it's being thrown, it can hit multiple targets as it involves two boomerangs being thrown and returning on the arcs where the other was. A lot of damage can come out of it if hit right at the end of the curve of the two boomerangs...to say nothing of the damage that you'll also take from being stunned for the duration.
The Vanguard's Battle Shout used to give him Rage stacks based off enemies hit and heal based on how much health was lost. Combined with the slow of the ability in addition to a trait to reduce the cost of EX abilities, the Vanguard was probably going to simultaneously kill you, have himself and his team not ever die and stop you from trying to avoid the former.