Hostile mobs on peacefull

Put the summary of the bug you're having here

*What I expected to happen was not to have hostile mobs spawn one peaceful

*What actually happened was a skeleton spawned
I was testing the new elytra on peaceful, when I saw a skeleton with no AI just stand in the dark.

Don't know how to trigger it, i was just going around in a custom world with a lot of high places. Ground level was 217. And I had Night Vision