"What happens is, a DK afflicts my priest with necrotic. Then I shield myself and heal. Should either the glyph of PW:S crit or heal crit ( or I use Prayer of Healing ) Divine Aegis procs as it should. Damage is then absorbed by the amount that divine Aegis states. However, once the damage absorption from Divine Aegis is spent the buff remains with the tool tip "Absorption amount remaining 0". This remains for it's duration of 12 seconds or unless otherwise refreshed.
During the time this nil value Divine Aegis is active, damage taken ignores the PW:S on the priest if he has necrotic strike on him. This leads to the priest taking a reduction in hit points while the absorb amount remaining on the PW:S remains untouched until either Divine Aegis fades or Necrotic is removed/falls off.
This bug can be recreated by having a Death Knight hit you with Necrotic Strike and keeping it refreshed. Casting PW:S on yourself and Prayer of Healing to proc Divine Aegis. Damage will then pass through the shield once Divine Aegis hits 0 leaving the amount of absorb remaining untouched on PW:S until one of the other two drops.
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Taken from EU priest forums