Nether portal spawning zombie pigmen.

I build a nether portal on top of my home and a zombie pigmen spawned out of it. It has not rained in about 3 Minecraft days so lighting couldn't have hit a pig. Also a pig couldn't have gotten on top of my house.

I would expect it not to spawn out of the nether portal.

I heard it and I looked on top of my house and the zombie pigmen was walking out of it.

1. I'm not sure how to trigger this bug. I just build a nether portal on top of my house in the jungle and it happened.
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