Nostalrius Begins - Quality wow vanilla realm (1.12)

Shadow Vulnerability is being consumed by Corruption ticks, when the tooltip for the Improved Shadow Bolt talent clearly states that it shouldn't. The bug occurs in a random fashion.

Your Shadow Bolt critical strikes increase Shadow damage dealt to the target by (4%/8%/12%/16/%20%) until 4 non-periodic damage sources are applied. Effect lasts a maximum of 12 sec.

*Edit* Curse of Agony also appears to have this consuming effect.

The first tick of drain life/drain soul also appears to be doing this. Only the first tick seems to consume the charge, and if SV appears after corruption/coa/etc are already applied, charges are not consumed.

Looks like you're right nervous, the charges are never consumed if the debuff is applied after shadow damage dots or drains are on the target, but only vice versa.