Lightning can give AI to pigs or villagers who originally had the NoAI NBT tag set to 1.
What I expected to happen was...:
The AI-less pigs or villagers stay at their respective forms when they are struck by lightning.
What actually happened was...:
The AI-less pig or villager actually changed into a zombie pigman or witch with AI.
Steps to Reproduce:
1. Summon a pig or villager using /summon ~ ~ ~ Pig {NoAi:1} or /summon ~ ~ ~ Villager {NoAI:1}
2. Summon a lightning bolt using /summon LightningBolt.
3. The two mobs become a witch or zombie pigman with AI.
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Pigs and villagers struck by lightning also lose their names.

Also happens when a zombie infects a villager or when a Zombie Villager is cured into a Villager.

Regressed in 15w38b, see MC-88967

You say "regressed" but the Tags tag didn't exist when this bug was created/fixed. Plus the title refers to NoAI which is still fixed in 15w38b.
Can confirm in 14w34d.