FeenixServerProject/Phoenix_1.12_Bugtracker — Issue #713: Hunter - Viper Sting
Author: Aleksandru
Created: 2012-02-18 13:51:05+00:00
State: open

according to wowwiki:

Patch 1.4.0 (2005-05-05): In addition, while it can be used on a druid target at any time it will only drain mana when the druid is in caster form.

it does not work like that here, if you try to use it on a druid while he is in cat or bear form, you wont be able to.

http://www.wowwiki.com/Viper_Sting#Patch_changes