If you use ming numbing poison or curse of tongues on a priest/warlock in pvp then their dots that benefit from haste (Shadow Word: Pain, Vampiric Touch, Devouring Plague, Corruption) will do 30% slower damage.
EX:
Vampiric Touch is 15 seconds duration. Damage tick every 3 seconds.
With Mind Numbing or Curse of Tongues this will change to:
Vampiric Touch 20 seconds duration. Damage tick every 4 seconds.
Basically you reduce all of their dot damage by 30%, regardless of haste.
Abuse before fix.